fix(sp27): Claim.patient_control_number populated from CLM01 (claim_id), not 2010BA NM109 (member_id)

The 837 ingest path (_claim_837_row in store.py:194) populated
Claim.patient_control_number from claim.subscriber.member_id — the
subscriber's 2010BA NM109 Medicaid ID — instead of from
claim.claim_id (CLM01, the claim submitter's identifier the 837
actually sent).

That silently broke every downstream join that uses this column as a
cross-reference key:

  * reconcile.match() (reconcile.py:74) — joins
    Claim.patient_control_number against
    Remittance.payer_claim_control_number (which is parsed from CLP01,
    the 835's echo of CLM01 per X12 spec). Member_id never matches
    CLP01, so auto-match always fails.
  * apply_999_rejections — same lookup, same broken key.
  * apply_277ca_rejections — same lookup, same broken key.
  * scoring.score_pair — same broken key.

Live DB probe (1,739 remits, 337 claims):
  * Claim.id == Remit.payer_claim_control_number    → 0 matches
  * Claim.patient_control_number == Remit.payer_claim_control_number
    → 9 matches (substring coincidences; synthetic PCN strings share
    alphanumeric characters with the human-readable member_ids)
  * Claim.matched_remittance_id NOT NULL             → 0 claims

This commit changes _claim_837_row to write
Claim.patient_control_number = claim.claim_id (= CLM01). The
reconcile matcher's existing join now hits the row the 835 echoes
back. Companion migration 0017 backfills the 337 pre-fix rows so
they're on equal footing with new ingests (UPDATE claims SET
patient_control_number = id WHERE patient_control_number IS DISTINCT
FROM id — idempotent).

Tests:
  * 2 new RED→GREEN tests in test_store_reconcile.py:
    - test_837_ingest_populates_patient_control_number_from_claim_id
      pins the field semantics directly
    - test_837_then_835_with_echoed_pcn_auto_pairs proves the full
      end-to-end auto-match now fires
  * 3 existing manual_match tests that relied on the bug (had setup
    using member_id != claim_id to deliberately prevent auto-match)
    updated to use distinct PCNs explicitly so the tests still
    exercise the manual-match path with a real orphan pair.
  * 3 store_claim_detail / api_gets tests adjusted for the same
    reason.

Verified: 1,176 / 1,177 backend tests pass; the one failure is the
pre-existing flake in test_provider_extended_response.py noted before
this work started.

Caveat for the existing dev DB: the 1,739 remits already in the DB
were ingested from 835 fixtures whose CLP01 is a different synthetic
identifier than the 837's CLM01 (the test fixtures never echoed
CLM01 — a fixture-data limitation, not a code bug). After this fix,
new 837+835 ingest pairs whose payer echoes CLM01 in CLP01 will
auto-match as expected. The pre-existing 1,739 remits will continue
to land in the unmatched bucket; that can only be fixed by
regenerating the test fixtures (out of scope for this SP).

backend/tests/test_db_migrate.py

@@ -121,14 +121,16 @@ def test_migration_latest_idempotent_on_fresh_db(tmp_path: Path) -> None:
     expected head from 14 to 15 with the new UNIQUE-drop migration.
     SP27 Task 11 bumped it to 16 with the claims.matched_remittance_id
     index (drift-check perf).
+    SP27 Task 17 bumped it to 17 with the patient_control_number
+    backfill UPDATE (the migration runner now applies DML too).
     """
     engine = _fresh_engine(tmp_path)
     db_migrate.run(engine)
     v_after_first = _user_version(engine)
-    assert v_after_first == 16, f"expected head=16, got {v_after_first}"
+    assert v_after_first == 17, f"expected head=17, got {v_after_first}"
 
     db_migrate.run(engine)
-    assert _user_version(engine) == 16, "second run should not bump version"
+    assert _user_version(engine) == 17, "second run should not bump version"
 
 
 def test_drop_claims_unique_constraint_migration(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
@@ -154,7 +156,7 @@ def test_drop_claims_unique_constraint_migration(tmp_path: Path, monkeypatch: py
     engine = _fresh_engine(tmp_path)
 
     db_migrate.run(engine)
-    assert _user_version(engine) == 16, f"expected head=16, got {_user_version(engine)}"
+    assert _user_version(engine) == 17, f"expected head=17, got {_user_version(engine)}"
 
     # Two claims in one batch with the same patient_control_number
     # must be insertable. If 0015's table recreation re-introduced a