feat(tags): system tags — is_system column, seeded hygiene tags, protection guards

Training hygiene step 1 (milestone #128). Migration 0075 adds
tag.is_system and seeds wip / banner / editor screenshot (kind=general),
ADOPTING an existing same-(name,kind) tag case-insensitively instead of
duplicating. These rows drive the upcoming training exclusions, so they
are protected: rename and merge-away refuse system tags (merge-INTO
stays allowed — folding an operator's old hygiene tag into the system
row is the intended move; merge is the only tag-delete path, so that
guard covers deletion). is_system rides every tag serialization.

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

tests/test_system_tags.py

@@ -0,0 +1,81 @@
+"""System tags (milestone #128): seeded rows, protection guards, serialization.
+
+The three hygiene tags (wip / banner / editor screenshot) ship via migration
+0075 and drive the training-hygiene exclusions. They must exist after
+migration, serialize with is_system, and refuse rename/merge-away — the
+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 backend.app.models import Tag, TagKind
+
+pytestmark = pytest.mark.integration
+
+SYSTEM_NAMES = {"wip", "banner", "editor screenshot"}
+
+
+async def _system_tag(db, name):
+    return (await db.execute(
+        select(Tag).where(Tag.is_system.is_(True), Tag.name == name)
+    )).scalar_one()
+
+
+@pytest.mark.asyncio
+async def test_seeded_system_tags_exist(db):
+    rows = (await db.execute(
+        select(Tag.name, Tag.kind).where(Tag.is_system.is_(True))
+    )).all()
+    assert {r.name for r in rows} == SYSTEM_NAMES
+    assert all(r.kind == TagKind.general for r in rows)
+
+
+@pytest.mark.asyncio
+async def test_get_tag_serializes_is_system(client, db):
+    wip = await _system_tag(db, "wip")
+    resp = await client.get(f"/api/tags/{wip.id}")
+    assert resp.status_code == 200
+    assert (await resp.get_json())["is_system"] is True
+
+
+@pytest.mark.asyncio
+async def test_rename_system_tag_refused(client, db):
+    wip = await _system_tag(db, "wip")
+    resp = await client.patch(
+        f"/api/tags/{wip.id}", json={"name": "work in progress"}
+    )
+    assert resp.status_code == 400
+    body = await resp.get_json()
+    assert "system tag" in body["error"]
+
+
+@pytest.mark.asyncio
+async def test_merge_away_system_tag_refused(client, db):
+    banner = await _system_tag(db, "banner")
+    created = await client.post(
+        "/api/tags", json={"name": "ad banner", "kind": "general"}
+    )
+    target_id = (await created.get_json())["id"]
+    resp = await client.post(
+        f"/api/tags/{banner.id}/merge", json={"target_id": target_id}
+    )
+    assert resp.status_code == 400
+    body = await resp.get_json()
+    assert "system tag" in body["error"]
+
+
+@pytest.mark.asyncio
+async def test_merge_into_system_tag_allowed(client, db):
+    wip = await _system_tag(db, "wip")
+    created = await client.post(
+        "/api/tags", json={"name": "old wip", "kind": "general"}
+    )
+    source_id = (await created.get_json())["id"]
+    resp = await client.post(
+        f"/api/tags/{source_id}/merge", json={"target_id": wip.id}
+    )
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["target"]["id"] == wip.id
+    assert body["source_deleted"] is True

tests/test_tag_query.py

@@ -10,20 +10,29 @@ def test_serialize_tag_with_enum_kind():
     # ORM/column rows carry kind as a TagKind enum.
     row = SimpleNamespace(
         id=1, name="Sasuke Uchiha", kind=TagKind.character,
-        fandom_id=5, fandom_name="Naruto",
+        fandom_id=5, fandom_name="Naruto", is_system=False,
     )
     assert serialize_tag(row) == {
         "id": 1, "name": "Sasuke Uchiha", "kind": "character",
-        "fandom_id": 5, "fandom_name": "Naruto",
+        "fandom_id": 5, "fandom_name": "Naruto", "is_system": False,
     }
 
 
 def test_serialize_tag_with_string_kind_and_no_fandom():
-    # autocomplete hits already store kind as a plain string.
+    # autocomplete hits already store kind as a plain string. A row from a
+    # select that predates the is_system column serializes as non-system.
     row = SimpleNamespace(
         id=2, name="solo", kind="general", fandom_id=None, fandom_name=None,
     )
     assert serialize_tag(row) == {
         "id": 2, "name": "solo", "kind": "general",
-        "fandom_id": None, "fandom_name": None,
+        "fandom_id": None, "fandom_name": None, "is_system": False,
     }
+
+
+def test_serialize_tag_system_row():
+    row = SimpleNamespace(
+        id=3, name="wip", kind=TagKind.general,
+        fandom_id=None, fandom_name=None, is_system=True,
+    )
+    assert serialize_tag(row)["is_system"] is True