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/007_card_events.sql

@@ -0,0 +1,11 @@
+-- Eventos cronologicos por card. Complementa column_history (moves) y lock_history (locks).
+-- Captura: created, assigned, unassigned, title_changed, description_changed, color_changed, tags_changed.
+CREATE TABLE IF NOT EXISTS card_events (
+    id         TEXT PRIMARY KEY,
+    card_id    TEXT NOT NULL REFERENCES cards(id) ON DELETE CASCADE,
+    kind       TEXT NOT NULL,
+    actor_id   TEXT,
+    payload    TEXT NOT NULL DEFAULT '{}',
+    created_at TEXT NOT NULL
+);
+CREATE INDEX IF NOT EXISTS idx_card_events_card ON card_events(card_id, created_at);