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

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")