feat(kanban): badges done/locked + drag locked en mismo column + migrations + UX stickers

- badges: locked → tiempo bloqueado; done → fecha completion + total lead time; otherwise → tiempo en columna
- locked cards: drag permitido dentro de mismo column (cross-column rejected con notification)
- card field: locked_at desde JOIN card_lock_history (open period)
- migrations: refactor a embed.FS, archivos 002-005 extraidos de ensureColumns; ensureColumns queda como backstop
- stickers UX: opacidad 1, debajo del texto, picker estable (useRef), boton entra directo a modo con 😀, popover cierra outside, cards done filter brightness
- format: formatDateTimeShort

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/format.ts

@@ -28,3 +28,14 @@ export function formatDuration(ms: number): string {
   const w = Math.floor((ms % MONTH) / WEEK);
   return w === 0 ? `${m}M` : `${m}M ${w}S`;
 }
+
+export function formatDateTimeShort(iso: string): string {
+  const d = new Date(iso);
+  if (Number.isNaN(d.getTime())) return "";
+  const dd = String(d.getDate()).padStart(2, "0");
+  const mm = String(d.getMonth() + 1).padStart(2, "0");
+  const yy = String(d.getFullYear()).slice(-2);
+  const hh = String(d.getHours()).padStart(2, "0");
+  const mi = String(d.getMinutes()).padStart(2, "0");
+  return `${dd}/${mm}/${yy} ${hh}:${mi}`;
+}