fix(fc3a): ruff I001 import-block fixes + use sqlalchemy.case (not func.case)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/services/artist_service.py

@@ -8,7 +8,7 @@ Dates come from Post.post_date via ImageProvenance.post_id.
 
 from dataclasses import dataclass
 
-from sqlalchemy import and_, func, or_, select
+from sqlalchemy import and_, case, func, or_, select
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -228,7 +228,7 @@ class ArtistService:
         like = f"%{cleaned.lower()}%"
         prefix_like = f"{cleaned.lower()}%"
         # Rank: exact (0) < prefix (1) < substring (2).
-        rank = func.case(
+        rank = case(
             (func.lower(Artist.name) == cleaned.lower(), 0),
             (func.lower(Artist.name).like(prefix_like), 1),
             else_=2,