fix(cleanup): don't flag a character's fandom (or a chaptered series) as unused

find_unused_tags only excluded tags with image_tag or series_page references, so
it flagged every fandom as 'unused' — fandoms are NEVER applied to images (a
character carries its fandom via tag.fandom_id), and the FK is ondelete=SET NULL,
so deleting one silently strips the fandom off all its characters
(operator-flagged 2026-06-08: artist-OC fandoms showing as unused).

Exclude tags referenced as a character's fandom_id, and (same class of gap) tags
referenced by a series_chapter (an all-placeholder series has chapters but no
pages yet). A genuinely orphaned fandom with no characters is still swept.

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

tests/test_cleanup_service.py

@@ -155,6 +155,31 @@ def test_find_unused_tags_returns_only_unreferenced(db_sync, tmp_path):
     assert names.index("aaa-unused") < names.index("zzz-unused")
 
 
+def test_find_unused_tags_excludes_fandom_referenced_by_character(db_sync):
+    # A fandom is never on an image — a character carries it via fandom_id — so
+    # an assigned fandom must NOT be flagged unused (deleting it SET-NULLs the
+    # fandom off its characters). Operator-flagged 2026-06-08.
+    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
+    orphan_fandom = _make_tag(db_sync, name="NobodysFandom", kind=TagKind.fandom)
+    db_sync.commit()
+
+    names = [t.name for t in cleanup_service.find_unused_tags(db_sync)]
+    assert "Creux" not in names          # referenced by a character → kept
+    assert "NobodysFandom" in names      # genuinely orphaned → still swept
+
+
+def test_find_unused_tags_excludes_series_with_only_chapters(db_sync):
+    # An all-placeholder series has chapters but no pages yet — not unused.
+    series = _make_tag(db_sync, name="EmptySeries", kind=TagKind.series)
+    db_sync.add(SeriesChapter(series_tag_id=series.id, chapter_number=1))
+    db_sync.commit()
+
+    names = [t.name for t in cleanup_service.find_unused_tags(db_sync)]
+    assert "EmptySeries" not in names
+
+
 # --- unlink_image_files ---------------------------------------------