feat: Implement junior fees dashboard and reconciliation

- Add dual-sheet architecture to pull attendance from both adult and junior spreadsheets.
- Introduce parsing rules to isolate juniors (e.g. above '# Treneri', tier 'J').
- Add new endpoints `/fees-juniors` and `/reconcile-juniors` to track junior attendances and match bank payments.
- Display granular attendance components showing adult vs. junior practices.
- Add fee rule configuration supporting custom pricing exceptions for specific months (e.g. Sep 2025) and merging billing periods.
- Add `make sync-2025` target to the Makefile for convenience.
- Document junior fees implementation logic and rules in prompts/outcomes.

Co-authored-by: Antigravity <antigravity@google.com>

scripts/match_payments.py

@@ -389,7 +389,7 @@ def reconcile(
     final_balances: dict[str, int] = {}
     for name in member_names:
         window_balance = sum(
-            int(mdata["paid"]) - mdata["expected"] 
+            int(mdata["paid"]) - (mdata["expected"] if isinstance(mdata["expected"], int) else 0)
             for mdata in ledger[name].values()
         )
         final_balances[name] = window_balance + credits.get(name, 0)