fix(835): drop over-constraining UNIQUE constraints, add multi-BPR warning

Three related changes for real CO Medicaid data:

1. Drop UNIQUE(batch_id, patient_control_number) on claims and
   UNIQUE(batch_id, payer_claim_control_number) on remittances. The X12
   spec allows multiple CLM segments per 2000B subscriber loop and 835
   ERAs can repeat a payer_claim_control_number for reversals. Claim/
   remittance identity is provided by the primary key (claims.id = CLM01,
   remittances.id = CLP01).

2. Add validator rule R835_MULTI_BPR warning for files with multiple BPR
   segments (CO Medicaid split-payment pattern). The parser already sums
   BPR02 paid_amounts; this surfaces the non-standard data to operators.

3. Skip R835_BAL_BPR_vs_CLP04 when BPR01='I' (Information Only 835).
   In that mode BPR02 is informational and the per-claim CLP04 totals
   are authoritative — a diff is expected, not an error.

Migration 0003 handles the drop with IF EXISTS so fresh DBs skip cleanly.
Updates affected tests to reflect new schema (no UNIQUE constraint on
batch_id + patient_control_number / payer_claim_control_number).

Fixes test_api_835::test_prodfile_round_trip_persists_separately which
was failing on real production data.

backend/tests/test_acks.py

@@ -49,17 +49,18 @@ def test_migration_0002_creates_acks_table():
     assert expected <= cols  # spec columns are all present
 
 
-def test_migration_0002_idempotent_on_fresh_db():
+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 2)."""
+    user_version already at the latest version — currently 3 after the
+    0003 constraint drop was added)."""
     with db.engine().begin() as c:
         v1 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v1 == 2
+    assert v1 == 3
     # 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 == 2
+    assert v2 == 3
 
 
 def test_add_ack_persists_row():