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/gallery_service.py

@@ -14,7 +14,6 @@ Decoding rejects malformed cursors with a ValueError; the API layer
 translates that to HTTP 400.
 """
 
-import base64
 from dataclasses import dataclass
 from datetime import datetime
 
@@ -24,8 +23,7 @@ from sqlalchemy.orm import aliased
 
 from ..models import Artist, ImageProvenance, ImageRecord, Post, Source, Tag
 from ..models.tag import image_tag
-
-CURSOR_SEPARATOR = "|"
+from .pagination import decode_cursor, encode_cursor
 
 # Reserved `platform` filter value selecting images with NO platformed
 # provenance (filesystem imports). Returned by facets() as a null-valued
@@ -35,20 +33,6 @@ CURSOR_SEPARATOR = "|"
 UNSOURCED_PLATFORM = "__unsourced__"
 
 
-def encode_cursor(effective_date: datetime, image_id: int) -> str:
-    raw = f"{effective_date.isoformat()}{CURSOR_SEPARATOR}{image_id}"
-    return base64.urlsafe_b64encode(raw.encode()).decode()
-
-
-def decode_cursor(cursor: str) -> tuple[datetime, int]:
-    try:
-        raw = base64.urlsafe_b64decode(cursor.encode()).decode()
-        ts_part, id_part = raw.split(CURSOR_SEPARATOR, 1)
-        return datetime.fromisoformat(ts_part), int(id_part)
-    except Exception as exc:
-        raise ValueError(f"invalid cursor: {cursor!r}") from exc
-
-
 def _effective_date_col():
     """The materialized gallery sort key: image_record.effective_date
     (alembic 0035) = COALESCE(primary post's post_date, created_at),