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>

tests/test_db_helpers.py

@@ -0,0 +1,34 @@
+"""Race-safe get_or_create helper (shared by the async services)."""
+
+import pytest
+from sqlalchemy import select
+
+from backend.app.models import Artist
+from backend.app.services.db_helpers import get_or_create
+
+pytestmark = pytest.mark.integration
+
+
+@pytest.mark.asyncio
+async def test_get_or_create_creates_then_returns_existing(db):
+    stmt = select(Artist).where(Artist.slug == "goc-test")
+
+    factory_calls = {"n": 0}
+
+    async def _make():
+        factory_calls["n"] += 1
+        a = Artist(name="GOC Test", slug="goc-test")
+        db.add(a)
+        await db.flush()
+        return a
+
+    row1, created1 = await get_or_create(db, stmt, _make)
+    assert created1 is True
+    assert row1.slug == "goc-test"
+    assert factory_calls["n"] == 1
+
+    # Second call finds the existing row and does NOT invoke the factory.
+    row2, created2 = await get_or_create(db, stmt, _make)
+    assert created2 is False
+    assert row2.id == row1.id
+    assert factory_calls["n"] == 1  # factory not called when the row exists