feat(ta1): TA1 interchange ACK parser, persistence, and API

Adds full TA1 (Interchange Acknowledgment, X12 envelope-level) support
to mirror the existing 999 transaction-set ACK pipeline.

Parser & models
- parsers/models_ta1.py: Pydantic Ta1Ack + ParseResultTa1 models
  (AckCode = Literal['A','E','R'], date serializer mirroring 999)
- parsers/parse_ta1.py: TA1 segment parser
  - Tolerant YYMMDD (6-digit) + CCYYMMDD (8-digit) date handling
    for CO Medicaid interchange_date / ack_generated_date
  - 106-char ISA validation (15-char sender/receiver IDs)
  - source_batch_id = 'TA1-<ISA13>'

Persistence
- migrations/0005_create_ta1_acks.sql: ta1_acks table + indexes
  on source_batch_id and ack_code
- db.py: Ta1Ack ORM model (flat columns + raw_json, mirrors Ack)
- store.py: add_ta1_ack, list_ta1_acks (returns all rows),
  get_ta1_ack
- db version bumped 4 -> 5 (test_acks.py updated)

API (api.py)
- POST /api/parse-ta1: text/file ingest, persists ta1_ack,
  returns detail-ready payload
- GET /api/ta1-acks: list with limit + total (mirrors 999 pattern)
- GET /api/ta1-acks/{ack_id}: detail
- _ta1_to_ui / _serialize_ta1 / _serialize_ta1_from_row helpers

Tests (17 new, 508 total passing)
- tests/test_parse_ta1.py (8): CCYYMMDD + YYMMDD acceptance,
  E/R codes, source_batch_id, missing ISA/TA1, short defaults
- tests/test_api_ta1.py (9): happy path, rejected persists,
  empty/malformed 400, missing file 422, detail regenerates
  segment, 404, empty list, newest-first
- tests/test_prodfiles_smoke.py: extended to smoke-test 352
  production TA1 files (A=0, R=352, E=0)
- tests/test_api_parse_persists.py: cross-pipeline reconciliation
  test asserting invariants across 837P + 835 prod files

Real-data finding: all 352 production TA1s are R (rejected);
operator follow-up warranted.

backend/tests/test_acks.py

@@ -51,16 +51,17 @@ 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 4 after the
-    0004 rejection columns + state-history index was added by SP6)."""
+    user_version already at the latest version — currently 5 after the
+    0004 rejection columns + state-history index (SP6) and the 0005
+    ta1_acks table (this PR)."""
     with db.engine().begin() as c:
         v1 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v1 == 4
+    assert v1 == 5
     # 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 == 4
+    assert v2 == 5
 
 
 def test_add_ack_persists_row():