fix: Distribute multi-month payments by per-month expected fee

reconcile() previously split a multi-month payment evenly across months,
which falsely flagged months as underpaid when their expected fees
differed (e.g. 1250 CZK for 02+03+04 2026 with rates 750/350/150 was
shown as 416/month with two months red).

The allocation now runs per matched member: greedy when the share covers
the total expected (each month gets its expected fee, surplus -> credit),
proportional by expected fee otherwise. Out-of-window months keep the
previous even-split-to-credit behavior. 6 new test cases.

Also adds CHANGELOG.md and a changelog convention in CLAUDE.md.

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

scripts/match_payments.py

@@ -385,8 +385,7 @@ def reconcile(
                     })
             continue
 
-        num_allocations = len(matched_members) * len(matched_months)
-        per_allocation = amount / num_allocations if num_allocations > 0 else 0
+        member_share = amount / len(matched_members) if matched_members else 0
 
         for member_name, confidence in matched_members:
             if member_name not in ledger:
@@ -397,20 +396,64 @@ def reconcile(
                 unmatched.append(tx)
                 continue
 
-            for month_key in matched_months:
-                entry = {
-                    "amount": per_allocation,
-                    "date": tx["date"],
-                    "sender": tx["sender"],
-                    "message": tx["message"],
-                    "confidence": confidence,
-                }
-                if month_key in ledger[member_name]:
-                    ledger[member_name][month_key]["paid"] += per_allocation
-                    ledger[member_name][month_key]["transactions"].append(entry)
-                else:
-                    # Future month — track as credit
-                    credits[member_name] = credits.get(member_name, 0) + int(per_allocation)
+            in_window = [(m, ledger[member_name][m]["expected"]) for m in matched_months if m in ledger[member_name]]
+            out_of_window = [m for m in matched_months if m not in ledger[member_name]]
+
+            # Out-of-window months (outside display range): even split → credit, same as before.
+            n_total = len(matched_months)
+            if out_of_window and n_total > 0:
+                out_credit = member_share / n_total * len(out_of_window)
+                credits[member_name] = credits.get(member_name, 0) + int(out_credit)
+            else:
+                out_credit = 0.0
+
+            in_window_share = member_share - out_credit
+
+            if not in_window:
+                continue
+
+            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.
+                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
+                    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,
+                    })
+            else:
+                # Fallback: no expected fees (prepayment before attendance recorded); even split.
+                per_month = in_window_share / len(in_window)
+                for m, _ in in_window:
+                    ledger[member_name][m]["paid"] += per_month
+                    ledger[member_name][m]["transactions"].append({
+                        "amount": per_month,
+                        "date": tx["date"],
+                        "sender": tx["sender"],
+                        "message": tx["message"],
+                        "confidence": confidence,
+                    })
 
     # Calculate final total balances (window + off-window credits)
     final_balances: dict[str, int] = {}