feat(go): M6.6 — /qr, /sync-bank, /flush-cache, /version pages

- GET /qr: Czech QR Platba PNG; ports Python qr_code() exactly
  (account validation, amount clamping, * stripping, SPD format)
- GET /sync-bank: Fio sync → infer → cache flush with captured log
- GET+POST /flush-cache: form + action, shows deleted count
- GET /version: JSON alias of /api/version (Python parity)
- FlushCache() added to membership.Sources; wired through api.Handler
- web.ActionHandlers{BankSync} closure-based dep injection for sync
- New dep: github.com/skip2/go-qrcode
- TestQRBuildSPD (9 cases), TestServeQR, TestServeFlushCache{GET,POST},
  TestServeSync, TestServeVersion added

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

go/internal/web/templates/flush_cache.tmpl

@@ -1,5 +1,11 @@
 {{define "title"}}Flush Cache{{end}}
 {{define "content"}}
 <h1>Flush Cache</h1>
-<p class="description">Coming in M6.6</p>
+{{if .Flushed}}
+  <p class="status-ok">Cache flushed: {{.Deleted}} file(s) deleted.</p>
+{{end}}
+<p class="description">Deletes all cached data files so the next request fetches fresh data from Google Sheets.</p>
+<form method="POST" action="/flush-cache">
+  <button type="submit" class="btn">Flush Cache</button>
+</form>
 {{end}}