feat(fc3a): migration 0010 — unique(source.artist_id, platform, url)

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

alembic/versions/0010_fc3a_source_unique_artist_platform_url.py

@@ -0,0 +1,32 @@
+"""fc3a: unique(source.artist_id, source.platform, source.url)
+
+Revision ID: 0010
+Revises: 0009
+Create Date: 2026-05-20
+
+Enforces FC-3a's dedup invariant at the DB level. No backfill — no
+existing rows are expected to collide; if they do the migration will
+fail loudly (intended).
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "0010"
+down_revision: Union[str, None] = "0009"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_unique_constraint(
+        "uq_source_artist_platform_url",
+        "source",
+        ["artist_id", "platform", "url"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint(
+        "uq_source_artist_platform_url", "source", type_="unique"
+    )

tests/test_migration_0010.py

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+import pytest
+from sqlalchemy.exc import IntegrityError
+
+from backend.app.models import Artist, Source
+
+pytestmark = pytest.mark.integration
+
+
+@pytest.mark.asyncio
+async def test_duplicate_artist_platform_url_rejected(db):
+    artist = Artist(name="Alice", slug="alice")
+    db.add(artist)
+    await db.flush()
+    db.add(Source(
+        artist_id=artist.id, platform="patreon",
+        url="https://patreon.com/alice", enabled=True,
+    ))
+    await db.flush()
+    db.add(Source(
+        artist_id=artist.id, platform="patreon",
+        url="https://patreon.com/alice", enabled=True,
+    ))
+    with pytest.raises(IntegrityError):
+        await db.flush()
+
+
+@pytest.mark.asyncio
+async def test_same_url_under_different_artist_ok(db):
+    a = Artist(name="A", slug="a")
+    b = Artist(name="B", slug="b")
+    db.add_all([a, b])
+    await db.flush()
+    db.add(Source(artist_id=a.id, platform="patreon", url="https://x/y", enabled=True))
+    db.add(Source(artist_id=b.id, platform="patreon", url="https://x/y", enabled=True))
+    await db.flush()  # must NOT raise