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/index.js

@@ -24,6 +24,7 @@ import contactsRouter from './routes/contacts.js';
 import budgetRouter from './routes/budget.js';
 import weatherRouter from './routes/weather.js';
 import preferencesRouter from './routes/preferences.js';
+import remindersRouter from './routes/reminders.js';
 
 const log     = createLogger('Server');
 const logSync = createLogger('Sync');
@@ -165,6 +166,7 @@ app.use('/api/v1/contacts', contactsRouter);
 app.use('/api/v1/budget', budgetRouter);
 app.use('/api/v1/weather', weatherRouter);
 app.use('/api/v1/preferences', preferencesRouter);
+app.use('/api/v1/reminders', remindersRouter);
 
 // --------------------------------------------------------
 // Health-Check (für Docker)