feat(kanban): deadlines en cards (context menu, badges, calendario, history)

- migration 009 + columna deadline TEXT en cards
- backend: CardPatch.HasDeadline, eventos deadline_set/deadline_cleared
- KanbanCard: menu derecho con DatePicker, badge countdown con colores por ratio (azul>=50%, amarillo<50%, rojo<10%, red.9 overdue)
- App.tsx: filtro "Con deadline", handleSetCardDeadline optimista, jump-to-card + highlight
- CalendarView: popover por dia con seq_num + titulo, click navega a card en tablero
- HistoryModal: render eventos deadline_set/deadline_cleared
- .gitignore: *.log

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

backend/migrations/002_add_stickers.sql

@@ -0,0 +1,4 @@
+-- Add stickers column to cards. Idempotent ALTER pattern in db.go ensureColumns.
+-- Stickers persist as JSON array: [{"emoji":"🔥","x":0.5,"y":0.5}, ...]
+-- x, y in [0, 1] relative to card dimensions for resize survival.
+ALTER TABLE cards ADD COLUMN stickers TEXT NOT NULL DEFAULT '[]';