fix(reconcile): fill earliest month deficit first in multi-month allocations

Replace proportional split with a fill-first loop that allocates
min(remaining, deficit) to each matched month in user-supplied order,
where deficit = expected - already_paid. Prior transactions' contributions
are now properly accounted for, so a second payment on overlapping months
fills only what's still owed instead of splitting proportionally by total
expected. Surplus after all deficits are covered goes to the credit bucket.

Fixes: Matyáš Thér 200+550 showing 566/183 instead of 500/250.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/match_payments.py

@@ -468,26 +468,19 @@ def reconcile(
 
             total_expected = sum(e for _, e in in_window)
 
-            if total_expected > 0 and in_window_share >= total_expected:
-                # Greedy phase: payment covers all in-window fees; overflow → credit.
-                credits[member_name] = credits.get(member_name, 0) + int(in_window_share - total_expected)
-                for m, exp in in_window:
-                    alloc = float(exp)
-                    ledger[member_name][m]["paid"] += alloc
-                    ledger[member_name][m]["transactions"].append({
-                        "amount": alloc,
-                        "date": tx["date"],
-                        "sender": tx["sender"],
-                        "message": tx["message"],
-                        "confidence": confidence,
-                    })
-            elif total_expected > 0:
-                # Proportional phase: distribute in_window_share by each month's expected fee.
-                # Last month absorbs any float remainder so the sum equals in_window_share exactly.
+            if total_expected > 0:
+                # Fill-first: iterate in_window in matched_months order (chronological by
+                # convention from infer_payments.py), allocating min(remaining, deficit) to
+                # each month. Deficit is net of what prior transactions already paid, so a
+                # second payment on the same months correctly fills only what remains due.
+                # Any surplus after all deficits are covered goes to the credit bucket.
                 remaining = in_window_share
-                for i, (m, exp) in enumerate(in_window):
-                    alloc = remaining if i == len(in_window) - 1 else in_window_share * exp / total_expected
-                    remaining -= alloc
+                for m, exp in in_window:
+                    paid_so_far = ledger[member_name][m]["paid"]
+                    deficit = max(0.0, float(exp) - paid_so_far)
+                    alloc = min(remaining, deficit)
+                    if alloc <= 0:
+                        continue
                     ledger[member_name][m]["paid"] += alloc
                     ledger[member_name][m]["transactions"].append({
                         "amount": alloc,
@@ -496,6 +489,9 @@ def reconcile(
                         "message": tx["message"],
                         "confidence": confidence,
                     })
+                    remaining -= alloc
+                if remaining > 0:
+                    credits[member_name] = credits.get(member_name, 0) + int(remaining)
             else:
                 # Fallback: no expected fees (prepayment before attendance recorded); even split.
                 per_month = in_window_share / len(in_window)