fix(cleanup): live prune uses the same predicate as the preview (data loss)

The fandom/chapter exclusions added in fb05c5e only touched find_unused_tags
(the preview SAMPLE). prune_unused_tags re-implemented the predicate inline for
the dry-run COUNT and the live DELETE with only the image_tag + series_page
checks — so the preview showed a safe list of names while the delete removed
every fandom (and chaptered series). Operator-flagged 2026-06-08: real data loss
— assigned fandoms deleted, their characters SET-NULLed.

Extract _unused_tag_conditions() as the single source of truth and use it for
the preview, the count, AND the delete, so they can never diverge again. Added a
prune-commit test asserting the LIVE delete spares a character's fandom and a
chaptered series.

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

tests/test_cleanup_service.py

@@ -362,6 +362,30 @@ def test_prune_unused_tags_commit_deletes_them(db_sync, tmp_path):
     assert "bye" not in surviving_names
 
 
+def test_prune_unused_tags_commit_spares_fandom_and_chaptered_series(db_sync):
+    # The LIVE delete (not just the preview) must use the same predicate — it
+    # previously had only the image_tag/series_page checks and deleted every
+    # fandom the preview correctly excluded (operator-flagged 2026-06-08, real
+    # data loss). Both dry-run count and the commit must spare referenced tags.
+    fandom = _make_tag(db_sync, name="Creux", kind=TagKind.fandom)
+    char = _make_tag(db_sync, name="OcChar", kind=TagKind.character)
+    char.fandom_id = fandom.id
+    series = _make_tag(db_sync, name="EmptySeries", kind=TagKind.series)
+    db_sync.add(SeriesChapter(series_tag_id=series.id, chapter_number=1))
+    _make_tag(db_sync, name="GenuinelyUnused")
+    db_sync.commit()
+
+    # dry-run count agrees with the delete: only the 1 truly-unused tag.
+    assert cleanup_service.prune_unused_tags(db_sync, dry_run=True)["count"] == 1
+    result = cleanup_service.prune_unused_tags(db_sync, dry_run=False)
+    assert result["deleted"] == 1
+
+    surviving = db_sync.execute(select(Tag.name)).scalars().all()
+    assert "Creux" in surviving          # fandom referenced by a character
+    assert "EmptySeries" in surviving    # series referenced by a chapter
+    assert "GenuinelyUnused" not in surviving
+
+
 # --- reset_content_tagging ------------------------------------------