feat(tags): system-tag UI markers + full protection sweep (step 4 of #128)

UI: shield marker + tooltip on TagChip and TagCard; system tags hide
rename/merge/delete affordances (chip kebab entirely — set-fandom never
applies to their general kind; remove stays, un-tagging is normal use).
Aliases stay available: mapping model outputs ONTO a system tag is
useful. Directory cards carry is_system.

Every destructive path that could take out a system row is now guarded,
found by sweeping run 1891s off-by-three failures — each one was a
surface that would have eaten the seeded tags:
- prune-unused: predicate exempts is_system (they ship with zero
  applications and matched every unused condition)
- reset-content: predicate exempts is_system AND keeps their
  applications — hygiene flags describe the file, not content tagging
- admin tag DELETE: refused with system_tag error
- normalize_existing_tags: scan excludes is_system — canonicalization
  would recase wip -> Wip behind TagService.rename's guard, breaking
  the name-keyed presentation lookup

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/api/admin.py

@@ -156,6 +156,10 @@ async def tag_delete(tag_id: int):
             )
         except LookupError:
             return _bad("not_found", status=404)
+        except ValueError as exc:
+            # System tags (#128) — the training-hygiene machinery keys on
+            # these rows.
+            return _bad("system_tag", detail=str(exc))
     return jsonify(result)
 
 

backend/app/services/cleanup_service.py

@@ -17,7 +17,7 @@ from datetime import UTC, datetime, timedelta
 from pathlib import Path
 from typing import Any
 
-from sqlalchemy import delete, func, or_, select, update
+from sqlalchemy import and_, delete, func, or_, select, update
 from sqlalchemy.orm import Session, aliased
 
 from ..models import (
@@ -203,6 +203,9 @@ def _unused_tag_conditions() -> list:
         Tag.id.not_in(used_via_series),
         Tag.id.not_in(used_via_chapter),
         Tag.id.not_in(used_via_fandom),
+        # System tags (#128) ship with zero applications and must survive a
+        # prune — the training-hygiene machinery keys on the rows.
+        Tag.is_system.is_(False),
     ]
 
 
@@ -402,6 +405,8 @@ def delete_tag(session: Session, *, tag_id: int) -> dict:
     tag = session.get(Tag, tag_id)
     if tag is None:
         raise LookupError(f"tag id not found: {tag_id}")
+    if tag.is_system:
+        raise ValueError(f"'{tag.name}' is a system tag and cannot be deleted")
     associations_count = count_tag_associations(session, tag_id=tag_id)
     info = {"id": tag.id, "name": tag.name, "kind": tag.kind.value}
     session.delete(tag)
@@ -731,7 +736,10 @@ def reset_content_tagging(session: Session, *, dry_run: bool = False) -> dict:
     heads retrain from whatever the operator re-tags. (The API route gates the
     live run behind a preview-derived confirm token for exactly this reason.)
 
-    PRESERVED: fandom + series tags and their series_page ordering. CASCADE on
+    PRESERVED: fandom + series tags and their series_page ordering, AND the
+    system hygiene tags (#128) WITH their applications — the reset re-tags
+    CONTENT concepts, while wip/banner flags describe the file itself and
+    re-flagging hundreds of banners by hand would be pure loss. CASCADE on
     image_tag / tag_alias / tag_suggestion_rejection clears each deleted tag's
     applications + metadata. Tag.fandom_id is SET NULL, so deleting character
     tags never touches the fandom rows. Irreversible except via DB backup
@@ -744,7 +752,9 @@ def reset_content_tagging(session: Session, *, dry_run: bool = False) -> dict:
        "sample_names": [first 50],
        and on live runs "deleted": total}
     """
-    predicate = Tag.kind.in_(RESETTABLE_TAG_KINDS)
+    predicate = and_(
+        Tag.kind.in_(RESETTABLE_TAG_KINDS), Tag.is_system.is_(False)
+    )
     rows = session.execute(
         select(Tag.id, Tag.name, Tag.kind).where(predicate)
     ).all()

backend/app/services/tag_directory_service.py

@@ -107,6 +107,7 @@ class TagDirectoryService:
                 "kind": tag.kind.value if hasattr(tag.kind, "value") else tag.kind,
                 "fandom_id": tag.fandom_id,
                 "fandom_name": fandom_name,
+                "is_system": tag.is_system,
                 "image_count": int(image_count),
                 "preview_thumbnails": previews.get(tag.id, []),
             }

backend/app/services/tag_service.py

@@ -806,6 +806,10 @@ async def normalize_existing_tags(
     rows = (
         await session.execute(
             select(Tag.id, Tag.name, Tag.kind, Tag.fandom_id)
+            # System tags (#128) are exempt: their names are the keys the
+            # hygiene machinery matches on, and canonicalization would recase
+            # 'wip' → 'Wip' (this path bypasses TagService.rename's guard).
+            .where(Tag.is_system.is_(False))
         )
     ).all()
     groups = _group_existing_tags(rows)

frontend/src/components/discovery/TagCard.vue

@@ -13,9 +13,15 @@
       <div class="fc-tagcard__name">
         <template v-if="!editing">
           {{ card.name }}
+          <v-icon
+            v-if="card.is_system" size="14" icon="mdi-shield-outline"
+            class="fc-tagcard__system"
+            title="System tag — tagged items are excluded from training other concepts"
+          />
           <span v-if="card.kind === 'character' && card.fandom_name"
                 class="fc-tagcard__fandom">— {{ card.fandom_name }}</span>
           <v-icon
+            v-if="!card.is_system"
             class="fc-tagcard__edit" size="14" icon="mdi-pencil"
             @click.stop="startEdit"
           />
@@ -58,12 +64,18 @@
               prepend-icon="mdi-tag-multiple"
               @click="$emit('aliases', card)"
             />
+            <!-- System tags (#128): merge-away and delete are refused
+                 server-side — the hygiene machinery keys on the row — so
+                 don't offer them. Aliases stay (mapping model outputs ONTO
+                 a system tag is useful). -->
             <v-list-item
+              v-if="!card.is_system"
               title="Merge with…"
               prepend-icon="mdi-call-merge"
               @click="$emit('merge-with', card)"
             />
             <v-list-item
+              v-if="!card.is_system"
               title="Delete tag"
               prepend-icon="mdi-delete"
               base-color="error"
@@ -133,6 +145,7 @@ function submit() {
 .fc-tagcard__body { padding: 8px 12px; }
 .fc-tagcard__name { font-weight: 600; }
 .fc-tagcard__fandom { font-weight: 400; opacity: 0.7; }
+.fc-tagcard__system { opacity: 0.6; margin-left: 2px; }
 .fc-tagcard__meta {
   display: flex; align-items: center; justify-content: space-between;
   margin-top: 4px;

frontend/src/components/modal/TagChip.vue

@@ -10,14 +10,23 @@
       @click:close="$emit('remove', tag.id)"
     >
       <v-icon start size="x-small">{{ iconFor(tag.kind) }}</v-icon>
-      {{ tag.name }}<span
+      {{ tag.name }}<v-icon
+        v-if="tag.is_system" end size="x-small" class="fc-tag-chip__system"
+        title="System tag — tagged items are excluded from training other concepts"
+      >mdi-shield-outline</v-icon><span
         v-if="tag.fandom_id" class="fc-tag-chip__fandom"
         :title="tag.fandom_name ? `Fandom: ${tag.fandom_name}` : 'Has a fandom'"
       >→<template v-if="fandomLabel">&nbsp;{{ fandomLabel }}</template></span>
     </v-chip>
     <!-- Modal-safe kebab is baked into KebabMenu (this chip lives in the
-         teleported image modal — #711). -->
-    <KebabMenu class="fc-tag-chip__kebab" :label="`More actions for ${tag.name}`">
+         teleported image modal — #711). System tags hide it entirely: rename
+         is refused server-side (the hygiene machinery keys on the row) and
+         set-fandom never applies to their general kind. Remove (✕) stays —
+         un-tagging an image is normal use. -->
+    <KebabMenu
+      v-if="!tag.is_system"
+      class="fc-tag-chip__kebab" :label="`More actions for ${tag.name}`"
+    >
       <v-list-item @click="$emit('rename', tag)">
         <v-list-item-title>Rename…</v-list-item-title>
       </v-list-item>
@@ -64,6 +73,7 @@ function iconFor (k) { return KIND_ICONS[k] || 'mdi-tag' }
   outline: 2px solid rgb(var(--v-theme-accent));
   outline-offset: 1px;
 }
+.fc-tag-chip__system { opacity: 0.6; margin-left: 2px; }
 .fc-tag-chip__kebab { opacity: 0.7; }
 .fc-tag-chip:hover .fc-tag-chip__kebab { opacity: 1; }
 .fc-tag-chip__fandom { opacity: 0.7; font-size: 0.85em; }

tests/test_system_tags.py

@@ -7,9 +7,14 @@ mechanism keys on these exact rows. Merge-INTO stays allowed: adopting an
 operator's old hygiene tag into the system row is the intended move.
 """
 import pytest
-from sqlalchemy import select
+from sqlalchemy import insert, select
 
-from backend.app.models import Tag, TagKind
+from backend.app.models import ImageRecord, Tag, TagKind
+from backend.app.models.tag import image_tag
+from backend.app.services.cleanup_service import (
+    prune_unused_tags,
+    reset_content_tagging,
+)
 
 pytestmark = pytest.mark.integration
 
@@ -65,6 +70,72 @@ async def test_merge_away_system_tag_refused(client, db):
     assert "system tag" in body["error"]
 
 
+@pytest.mark.asyncio
+async def test_admin_delete_system_tag_refused(client, db):
+    wip = await _system_tag(db, "wip")
+    resp = await client.delete(f"/api/admin/tags/{wip.id}")
+    assert resp.status_code == 400
+    body = await resp.get_json()
+    assert body["error"] == "system_tag"
+    still = (await db.execute(
+        select(Tag.id).where(Tag.id == wip.id)
+    )).scalar_one_or_none()
+    assert still == wip.id
+
+
+@pytest.mark.asyncio
+async def test_prune_unused_skips_system_tags(db):
+    """The seeded system tags have zero applications and would match every
+    'unused' condition — the shared predicate must exempt them (preview AND
+    live delete, same conditions)."""
+    preview = await db.run_sync(
+        lambda s: prune_unused_tags(s, dry_run=True)
+    )
+    assert preview["count"] == 0
+    assert preview["sample_names"] == []
+    await db.run_sync(lambda s: prune_unused_tags(s, dry_run=False))
+    remaining = (await db.execute(
+        select(Tag.name).where(Tag.is_system.is_(True))
+    )).scalars().all()
+    assert set(remaining) == SYSTEM_NAMES
+
+
+@pytest.mark.asyncio
+async def test_reset_content_preserves_system_tags_and_applications(db):
+    """Reset re-tags CONTENT concepts; hygiene flags describe the file itself
+    and survive with their applications."""
+    wip = await _system_tag(db, "wip")
+    plain = Tag(name="doomed concept", kind=TagKind.general)
+    db.add(plain)
+    img = ImageRecord(
+        path="/images/r.jpg", sha256="f" * 64, size_bytes=1, mime="image/jpeg",
+        width=1, height=1, origin="imported_filesystem",
+        integrity_status="unknown",
+    )
+    db.add(img)
+    await db.flush()
+    for tid in (wip.id, plain.id):
+        await db.execute(insert(image_tag).values(
+            image_record_id=img.id, tag_id=tid, source="manual",
+        ))
+    await db.commit()
+
+    result = await db.run_sync(
+        lambda s: reset_content_tagging(s, dry_run=False)
+    )
+    assert result["deleted"] == 1  # only the plain general tag
+    names_left = (await db.execute(
+        select(Tag.name).where(Tag.kind == TagKind.general)
+    )).scalars().all()
+    assert "doomed concept" not in names_left
+    assert "wip" in names_left
+    wip_apps = (await db.execute(
+        select(image_tag.c.image_record_id)
+        .where(image_tag.c.tag_id == wip.id)
+    )).scalars().all()
+    assert wip_apps == [img.id]
+
+
 @pytest.mark.asyncio
 async def test_merge_into_system_tag_allowed(client, db):
     wip = await _system_tag(db, "wip")