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>

CHANGELOG.md

@@ -1,5 +1,14 @@
 # Changelog
 
+## 2026-05-11 23:58 CEST — fix(reconcile): fill earliest month deficit first in multi-month allocations
+
+- Multi-month payment allocation now fills the earliest in-window deficit first and spills
+  any remainder to later months, accounting for prior transactions' contributions to each month.
+  Previously a single transaction was split proportionally to each month's total expected fee,
+  ignoring what earlier transactions had already paid — surfaced by Matyáš Thér's 200+550 case
+  showing 566/183 instead of 500/250.
+- Files: `scripts/match_payments.py`, `go/internal/domain/reconcile/reconcile.go`, tests, parity fixtures.
+
 ## 2026-05-11 22:56 CEST — fix(python): parse Fio 2-digit-year dates + add `make sync-debug` dry-run tool
 
 - Fix: `scripts/fio_utils.py` `parse_czech_date` now accepts `DD.MM.YY` / `D.M.YY` in addition to the 4-digit-year variants. Fio's transparent page now mixes both forms in the same response; the 2-digit rows were being silently dropped, which caused `make sync-2026` to miss every recent transfer. Mirrors the Go-side fix from 2026-05-07 (CHANGELOG entry below).