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/004_cards_extras.sql

@@ -0,0 +1,9 @@
+-- Columnas extra de `cards` (color, locked, assignee_id, completed_at, deleted_at, tags).
+-- Antes vivian en ensureColumns Go. La columna stickers va aparte en 002.
+ALTER TABLE cards ADD COLUMN color        TEXT    NOT NULL DEFAULT '';
+ALTER TABLE cards ADD COLUMN locked       INTEGER NOT NULL DEFAULT 0;
+ALTER TABLE cards ADD COLUMN assignee_id  TEXT;
+ALTER TABLE cards ADD COLUMN completed_at TEXT;
+ALTER TABLE cards ADD COLUMN deleted_at   TEXT;
+ALTER TABLE cards ADD COLUMN tags         TEXT    NOT NULL DEFAULT '[]';
+CREATE INDEX IF NOT EXISTS idx_cards_assignee ON cards(assignee_id);