refactor(services): shared pagination cursor (DRY sweep)

encode_cursor/decode_cursor (base64 <iso8601>|<id>) were defined identically in
gallery_service AND post_feed_service, with artist_service importing gallery's
copy. Two implementations of one cursor format silently break pagination in
whichever feed drifts. Extract to services/pagination.py; gallery/post_feed/
artist all import it. Dropped now-unused base64/datetime imports.

§8b: encode_cursor/decode_cursor now defined only in pagination.py. Existing
cursor round-trip tests still cover it via the re-export. Catalog updated.

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

backend/app/services/artist_service.py

@@ -24,7 +24,8 @@ from ..models import (
 from ..models.tag import image_tag
 from ..utils.slug import slugify
 from .db_helpers import get_or_create
-from .gallery_service import decode_cursor, encode_cursor, thumbnail_url
+from .gallery_service import thumbnail_url
+from .pagination import decode_cursor, encode_cursor
 
 
 @dataclass(frozen=True)