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"
+    )