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/009_card_deadline.sql

@@ -0,0 +1,4 @@
+-- Deadline opcional por card. Fecha RFC3339 (precision dia o instante).
+-- NULL = sin deadline (default). El frontend muestra countdown hasta la fecha.
+ALTER TABLE cards ADD COLUMN deadline TEXT;
+CREATE INDEX IF NOT EXISTS idx_cards_deadline ON cards(deadline) WHERE deadline IS NOT NULL;