feat(sp22): migration 0015 — drop inline UNIQUE on claims, add tests

Adds migration 0015_drop_claims_unique_constraint.sql that recreates the
claims table without the inline UNIQUE(batch_id, patient_control_number)
constraint, plus tests proving the migration runs cleanly and the
constraint is not re-introduced.

backend/tests/test_acks.py

@@ -51,20 +51,21 @@ def test_migration_0002_creates_acks_table():
 
 def test_migration_latest_idempotent_on_fresh_db():
     """Re-running the migration on the same DB must be a no-op (PRAGMA
-    user_version already at the latest version — currently 14 after
+    user_version already at the latest version — currently 15 after
     0004-0006 line_reconciliation, 0005 ta1_acks, SP9's 0007
     providers/payers/clearhouse, SP10's 0008 payer_rejected,
     SP11's 0009 audit_log, SP14's 0010 payer_rejected_acknowledged,
     SP16's 0011 processed_inbound_files, SP17's 0012 db_backups,
-    SP-auth's 0013 users + sessions, SP-audit's 0014 audit_log.user_id)."""
+    SP-auth's 0013 users + sessions, SP-audit's 0014 audit_log.user_id,
+    SP22's 0015 drop_claims_unique_constraint)."""
     with db.engine().begin() as c:
         v1 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v1 == 14
+    assert v1 == 15
     # A second run should not raise and should not bump the version.
     db_migrate.run(db.engine())
     with db.engine().begin() as c:
         v2 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v2 == 14
+    assert v2 == 15
 
 
 def test_add_ack_persists_row():