refactor: code quality improvements across the backend

- Remove insecure SSL verification bypass in attendance.py
- Add gunicorn as production WSGI server (Dockerfile + entrypoint)
- Fix silent data loss in reconciliation (log + surface unmatched members)
- Add required column validation in payment sheet parsing
- Add input validation on /qr route (account format, amount bounds, SPD injection)
- Centralize configuration into scripts/config.py
- Extract credentials path to env-configurable constant
- Hide unmatched transactions from reconcile-juniors page
- Fix test mocks to bypass cache layer (all 8 tests now pass reliably)
- Add pytest + pytest-cov dev dependencies
- Fix typo "Inffering" in infer_payments.py
- Update CLAUDE.md to reflect current project state

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/infer_payments.py

@@ -102,7 +102,7 @@ def infer_payments(spreadsheet_id: str, credentials_path: str, dry_run: bool = F
     member_names = [m[0] for m in members_data]
 
     # 3. Process rows
-    print("Inffering details for empty rows...")
+    print("Inferring details for empty rows...")
     updates = []
     
     for i, row in enumerate(rows[1:], start=2):