fix(sync): dashboard sync covers current year plus Nov-Dec of previous

/sync-bank now uses one shared default window in both implementations:
1 November of the previous year to 31 December of the current year.
Payments dated around New Year are picked up in January; duplicates
are still filtered by Sync ID.

Python used a hardcoded 2026 window and would have stopped syncing on
2027-01-01; Go used the current calendar year only.

Adds make sync-year [YEAR=N] with the same window. The CLI default
without flags (last 30 days) is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/plans/2026-10-08-1026-pre-feature-cleanup-tasks.md

@@ -50,7 +50,9 @@ before new feature work starts. Companion to the Go rewrite tracker,
   - September and October 2026 are billed at the defaults: 700 CZK adults,
     500 CZK juniors.
   - If those are not the intended rates, add the months in both files.
-- [ ] **T3** Remove the hardcoded 2026 window from Python `/sync-bank`
+- [x] **T3** Done 2026-10-08, extended: both dashboards now sync the
+  current year plus November–December of the previous one. Original task:
+  remove the hardcoded 2026 window from Python `/sync-bank`
   (`app.py`, the `sync_to_sheets` call with `2026-01-01` / `2026-12-31`).
   - It stops picking up transactions on 2027-01-01. Go already uses the
     current year.