refactor(services): shared race-safe get_or_create helper (DRY backend sweep)

The find-or-create dance — SELECT, then a SAVEPOINT INSERT that recovers (not a
full rollback) on IntegrityError when a concurrent worker inserted first — was
hand-rolled identically in 4 async sites: ArtistService.find_or_create,
TagService.find_or_create, ExtensionService._find_or_create_artist and
._find_or_create_source. Divergent copies of exactly this pattern are how the
duplicate-row/race bugs in reference_scalar_one_or_none_duplicates crept in, so
it now lives once in services/db_helpers.get_or_create (returns (row, created);
factory adds+flushes+returns the row; caller owns the outer commit).

Over-DRY guard: SourceService's IntegrityError sites RAISE DuplicateSourceError
(reject-on-conflict, a different concept) — left alone. Importer._get_or_create
is the lone SYNC consumer (already shared by 2 callers) — stays separate, can't
cross the sync/async boundary. §8b: no hand-rolled async find-or-create remains.
Test: get_or_create creates then returns existing without re-invoking the factory.

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

backend/app/services/artist_service.py

@@ -10,7 +10,6 @@ from dataclasses import dataclass
 
 from sqlalchemy import and_, case, func, or_, select
 from sqlalchemy.dialects.postgresql import insert as pg_insert
-from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from ..models import (
@@ -24,6 +23,7 @@ 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
 
 
@@ -250,12 +250,10 @@ class ArtistService:
         )
 
     async def find_or_create(self, name: str) -> tuple[Artist, bool]:
-        """Return (artist, created). Slug-keyed; idempotent under races.
-
-        Audit 2026-06-02: switched from session.rollback() to a
-        begin_nested savepoint + IntegrityError recovery so a lost
-        race doesn't unwind the calling request's surrounding work.
-        Mirrors importer._get_or_create.
+        """Return (artist, created). Slug-keyed; idempotent under races via the
+        shared race-safe db_helpers.get_or_create (savepoint + IntegrityError
+        recovery). A new artist also seeds an ArtistVisit so the directory's
+        `+N new` badge starts at 0.
         """
         cleaned = (name or "").strip()
         if not cleaned:
@@ -263,12 +261,8 @@ class ArtistService:
         slug = slugify(cleaned)
 
         select_existing = select(Artist).where(Artist.slug == slug)
-        existing = (await self.session.execute(select_existing)).scalar_one_or_none()
-        if existing is not None:
-            return existing, False
 
-        sp = await self.session.begin_nested()
-        try:
+        async def _create() -> Artist:
             artist = Artist(name=cleaned, slug=slug)
             self.session.add(artist)
             await self.session.flush()
@@ -279,13 +273,14 @@ class ArtistService:
             # count every image imported in the same session.
             self.session.add(ArtistVisit(artist_id=artist.id))
             await self.session.flush()
-            await sp.commit()
-        except IntegrityError:
-            await sp.rollback()
-            existing = (await self.session.execute(select_existing)).scalar_one()
-            return existing, False
-        await self.session.commit()
-        return artist, True
+            return artist
+
+        artist, created = await get_or_create(
+            self.session, select_existing, _create
+        )
+        if created:
+            await self.session.commit()
+        return artist, created
 
     async def autocomplete(self, prefix: str, limit: int = 20) -> list[Artist]:
         cleaned = (prefix or "").strip()