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>
