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>

backend/app/services/cleanup_service.py

@@ -21,6 +21,7 @@ from sqlalchemy import func, or_, select, update
 from sqlalchemy.orm import Session
 
 from ..models import Artist, ImageRecord, LibraryAuditRun, Tag
+from ..models.series_chapter import SeriesChapter
 from ..models.series_page import SeriesPage
 from ..models.tag import image_tag
 
@@ -141,21 +142,36 @@ def count_tag_associations(session: Session, *, tag_id: int) -> int:
 def find_unused_tags(
     session: Session, *, limit: int | None = None,
 ) -> list[Tag]:
-    """Tags with no image_tag rows AND no series_page rows.
+    """Tags genuinely referenced by nothing — safe to sweep.
 
-    Sorted by name. Used by both dry-run preview and the live prune.
-    A tag is "unused" iff it has zero rows in image_tag AND zero rows
-    in series_page (so we don't accidentally prune a series tag that
-    happens to have no images yet).
+    Sorted by name. Used by both dry-run preview and the live prune. A tag is
+    "unused" iff it has zero references across ALL the ways a tag can be in use:
+      - image_tag           (applied to an image)
+      - series_page         (a series tag with ordered pages)
+      - series_chapter      (a series tag with chapters but no pages yet, e.g.
+                             all-placeholder)
+      - tag.fandom_id       (a fandom referenced by a character)
+
+    The fandom check is essential: fandom tags are NEVER applied to images — a
+    character carries its fandom via fandom_id — so without it every assigned
+    fandom looked "unused", and the FK is ondelete=SET NULL, so deleting one
+    would silently strip the fandom off all its characters (operator-flagged
+    2026-06-08).
     """
     used_via_image_tag = select(image_tag.c.tag_id).distinct()
     used_via_series = select(SeriesPage.series_tag_id).where(
         SeriesPage.series_tag_id.is_not(None)
     ).distinct()
+    used_via_chapter = select(SeriesChapter.series_tag_id).distinct()
+    used_via_fandom = select(Tag.fandom_id).where(
+        Tag.fandom_id.is_not(None)
+    ).distinct()
     stmt = (
         select(Tag)
         .where(Tag.id.not_in(used_via_image_tag))
         .where(Tag.id.not_in(used_via_series))
+        .where(Tag.id.not_in(used_via_chapter))
+        .where(Tag.id.not_in(used_via_fandom))
         .order_by(Tag.name)
     )
     if limit is not None: