feat(fc3a): ArtistService.find_or_create + autocomplete

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

backend/app/services/artist_service.py

@@ -9,6 +9,7 @@ Dates come from Post.post_date via ImageProvenance.post_id.
 from dataclasses import dataclass
 
 from sqlalchemy import and_, func, or_, select
+from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from ..models import (
@@ -20,6 +21,7 @@ from ..models import (
     Tag,
 )
 from ..models.tag import image_tag
+from ..utils.slug import slugify
 from .gallery_service import decode_cursor, encode_cursor, thumbnail_url
 
 
@@ -191,3 +193,49 @@ class ArtistService:
             ],
             next_cursor=next_cursor,
         )
+
+    async def find_or_create(self, name: str) -> tuple[Artist, bool]:
+        """Return (artist, created). Slug-keyed; idempotent under races."""
+        cleaned = (name or "").strip()
+        if not cleaned:
+            raise ValueError("artist name must not be empty")
+        slug = slugify(cleaned)
+
+        existing = (await self.session.execute(
+            select(Artist).where(Artist.slug == slug)
+        )).scalar_one_or_none()
+        if existing is not None:
+            return existing, False
+
+        artist = Artist(name=cleaned, slug=slug)
+        self.session.add(artist)
+        try:
+            await self.session.flush()
+        except IntegrityError:
+            await self.session.rollback()
+            existing = (await self.session.execute(
+                select(Artist).where(Artist.slug == slug)
+            )).scalar_one()
+            return existing, False
+        await self.session.commit()
+        return artist, True
+
+    async def autocomplete(self, prefix: str, limit: int = 20) -> list[Artist]:
+        cleaned = (prefix or "").strip()
+        if not cleaned:
+            return []
+        like = f"%{cleaned.lower()}%"
+        prefix_like = f"{cleaned.lower()}%"
+        # Rank: exact (0) < prefix (1) < substring (2).
+        rank = func.case(
+            (func.lower(Artist.name) == cleaned.lower(), 0),
+            (func.lower(Artist.name).like(prefix_like), 1),
+            else_=2,
+        ).label("rank")
+        rows = (await self.session.execute(
+            select(Artist, rank)
+            .where(func.lower(Artist.name).like(like))
+            .order_by(rank, Artist.name.asc())
+            .limit(limit)
+        )).all()
+        return [a for a, _ in rows]