fix(sp41): 2010BB payer loop ordering + patient loop structure + visits table

**The bug.** The 837P X12 005010X222A1 IG requires the Payer Name
loop (2010BB / NM1*PR) to live INSIDE 2000B, AFTER the subscriber's
DMG and BEFORE 2000C (HL*3, the patient loop). The previous
serializer emitted NM1*PR AFTER the patient loop, which pyX12 caught
as 'Mandatory loop 2010BB missing' on round-trip. The 10/10 spot-check
files were therefore structurally invalid per the IG even though they
round-tripped through our own parser (which silently swallowed the
misplaced NM1*PR).

**The fix.**
1. Move 2010BB into 2000B in `_build_subscriber_block` so the order
   is now: HL*2 → SBR → NM1*IL → N3 → N4 → DMG → NM1*PR → HL*3
   → PAT → NM1*QC → N3 → N4 → DMG → CLM.
2. Add `_consume_patient_loop` to `parse_837.py` so the 2000B
   subscriber iteration skips past the 2000C patient loop and
   finds loop 2300 (CLM) — the parser was previously relying on
   the (invalid) old ordering.
3. Strip NM108/NM109 from NM1*QC (the IG marks these as 'Not Used')
   via the QC branch of `_build_nm1`.
4. Add PAT*01 segment (PAT is required whenever 2000C is emitted).
5. Add `include_patient_loop` flag so callers can opt out (e.g.
   Patient==Subscriber cases that don't need a 2000C loop at all).
6. Bump HL*2 child count to 1 when 2000C is emitted (was 0 or 1
   inconsistently).

**Visits table (gap #3).** Migration 0023 adds a `visits` table
holding the AxisCare visits export rows. The previous spot-check
driver read the CSV in-memory; persisting the roster makes the
source-of-truth reviewable via SQL. `cyclone.rebill.visits_store`
provides `load_visits_csv` (with UNIQUE-key dedup) and
`query_visits` (filterable read-back). 8 new unit tests pin the
contract.

**Misc.**
- guard `s.svc_date is not None` in `reconcile.run` against 835
  rows that lack DTP*472.
- `.gitignore`: dev/rebills/ (the 355k generated 837P files from
  pipeline-A runs) so they no longer pollute 'git status'.
- Bump migration-head assertions in test_db_migrate / test_acks /
  test_migration_0020 from 22 → 23.

**Verification (post-fix).**
- 1568/1568 unit tests pass (10s)
- 20/20 spot-check files pass pyX12 v4.0.0 (BSD-licensed local X12
  validator; the Edifabric live API is still quota-blocked)
- 10/10 batch-1 + 10/10 batch-2 files have all required segment
  classes (NM1*41, NM1*40, NM1*QC, NM1*IL, CLM*, SV1*HC:, DTP*472)
- spot-check.json + spot-check-summary.txt refreshed from
  pyX12 (was 10/10 stale 403 errors; now 20/20 PASS)

Refs: SP41 inwindow-rebill-pipeline, plan step 3 (10 well-formed 837P
files with NM1*QC patient/subscriber loop) + plan step 4 (Edifabric /
v2/x12/validate).

backend/tests/test_migration_0020.py

@@ -105,7 +105,7 @@ def migrated_engine(tmp_path: Path, monkeypatch: pytest.MonkeyPatch):
     # Confirm head is 22 (every migration applied, including SP41's 0022).
     with engine.connect() as conn:
         v = conn.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v == 22, f"expected migration head=22, got {v}"
+    assert v == 23, f"expected migration head=23, got {v}"
 
     yield engine
     engine.dispose()