feat: add reminders for tasks and calendar events (closes #13)

- DB migration #8: reminders table (entity_type, entity_id, remind_at, dismissed, created_by)
- REST API: GET /pending, GET /?entity, POST /, PATCH /:id/dismiss, DELETE
- Client polling module (reminders.js): 60s interval, toast + Browser Notification API
- Tasks: enable reminder with custom date/time in edit modal
- Calendar: reminder offset selector (at time / 15min / 1h / 1d before)
- Bell badge shows pending count; reminders auto-dismiss after 30s or on user action
- SW shell cache updated to include reminders.js + reminders.css
- 11 new DB tests covering CRUD, pending query, dismiss, upsert, cascade delete, constraints

server/db.js

@@ -369,6 +369,25 @@ const MIGRATIONS = [
       ALTER TABLE meal_ingredients ADD COLUMN category TEXT NOT NULL DEFAULT 'Sonstiges';
     `,
   },
+  {
+    version: 8,
+    description: 'Erinnerungen (Reminders) für Aufgaben und Kalender-Events',
+    up: `
+      CREATE TABLE IF NOT EXISTS reminders (
+        id          INTEGER PRIMARY KEY AUTOINCREMENT,
+        entity_type TEXT    NOT NULL CHECK(entity_type IN ('task', 'event')),
+        entity_id   INTEGER NOT NULL,
+        remind_at   TEXT    NOT NULL,
+        dismissed   INTEGER NOT NULL DEFAULT 0,
+        created_by  INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+        created_at  TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now'))
+      );
+
+      CREATE INDEX IF NOT EXISTS idx_reminders_entity ON reminders(entity_type, entity_id);
+      CREATE INDEX IF NOT EXISTS idx_reminders_remind ON reminders(remind_at);
+      CREATE INDEX IF NOT EXISTS idx_reminders_user   ON reminders(created_by);
+    `,
+  },
 ];
 
 /**