Add one-off tasks/appointments feature

- DB: tasks table with scheduled_datetime, reminder_minutes_before, advance_notified, status
- API: CRUD routes GET/POST /api/tasks, PATCH/DELETE /api/tasks/<id>
- Scheduler: check_task_reminders() fires advance + at-time notifications, tracks advance_notified to prevent double-fire
- Bot: handle_task() with add/list/done/cancel/delete actions + datetime resolution helper
- AI: task interaction type + examples added to command_parser
- Web: task list page with overdue/notified color coding + new task form with datetime-local picker
- Nav: replaced Templates with Tasks in bottom nav

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/schema.sql

@@ -298,3 +298,19 @@ CREATE INDEX IF NOT EXISTS idx_snitch_contacts_user ON snitch_contacts(user_uuid
 -- ── Migrations ──────────────────────────────────────────────
 -- Add IANA timezone name to user preferences (run once on existing DBs)
 ALTER TABLE user_preferences ADD COLUMN IF NOT EXISTS timezone_name VARCHAR(100);
+
+-- ── Tasks (one-off appointments/reminders) ──────────────────
+CREATE TABLE IF NOT EXISTS tasks (
+    id UUID PRIMARY KEY,
+    user_uuid UUID REFERENCES users(id) ON DELETE CASCADE,
+    title VARCHAR(255) NOT NULL,
+    description TEXT,
+    scheduled_datetime TIMESTAMP NOT NULL,
+    reminder_minutes_before INTEGER DEFAULT 15,
+    advance_notified BOOLEAN DEFAULT FALSE,
+    status VARCHAR(20) DEFAULT 'pending',
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+CREATE INDEX IF NOT EXISTS idx_tasks_user_scheduled ON tasks(user_uuid, scheduled_datetime);
+CREATE INDEX IF NOT EXISTS idx_tasks_pending ON tasks(status) WHERE status = 'pending';