feat(tags): 'Reset content tagging' admin action

Wipe every general + character tag so the operator can re-tag from scratch via
the Camie auto-suggest, while PRESERVING fandoms, series (+ series_page order),
and each image's stored tagger_predictions (so suggestions repopulate
immediately). One set-based DELETE FROM tag WHERE kind IN ('general','character')
— the five tag-referencing tables all cascade, so applications + aliases +
allowlist + rejections + centroids clear automatically; series tags aren't
deleted so series survive; Tag.fandom_id is SET NULL so fandoms are untouched.

Reuses the established dry-run-preview -> confirm pattern: cleanup_service.
reset_content_tagging() + POST /api/admin/tags/reset-content +
TagMaintenanceCard section with a backup-first warning and a red confirm
showing exact counts (tags by kind + image applications). Irreversible except
via DB backup restore; the wipe only fires when the operator confirms.

Tests: service dry-run counts + live delete preserves fandom/series/series_page
while content tags + their image_tag cascade away; API dry-run wiring.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_api_admin.py

@@ -435,3 +435,26 @@ async def test_trigger_vacuum_queues_the_task(client, monkeypatch):
     resp = await client.post("/api/admin/maintenance/vacuum")
     assert resp.status_code == 202
     assert calls == [1]
+
+
+# --- Tier-A: POST /tags/reset-content -------------------------------
+
+
+@pytest.mark.asyncio
+async def test_reset_content_tagging_dry_run_returns_counts(client, db):
+    db.add_all([
+        Tag(name="solo", kind=TagKind.general),
+        Tag(name="naruto", kind=TagKind.character),
+        Tag(name="Naruto", kind=TagKind.fandom),
+        Tag(name="my-series", kind=TagKind.series),
+    ])
+    await db.commit()
+    resp = await client.post(
+        "/api/admin/tags/reset-content", json={"dry_run": True}
+    )
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["count"] == 2
+    assert body["by_kind"] == {"general": 1, "character": 1}
+    # dry-run leaves the rows in place — fandom + series untouched too.
+    assert "deleted" not in body