feat: email notifications, webhook support, ICS export — closes #110

Email Notifications:
- SMTP configuration in Admin > Settings (host, port, user, pass, from)
- App URL setting for email CTA links
- Webhook URL support (Discord, Slack, custom)
- Test email button with SMTP validation
- Beautiful HTML email template with TREK logo, slogan, red heart footer
- All notification texts translated in 8 languages (en/de/fr/es/nl/ru/zh/ar)
- Emails sent in each user's language preference

Notification Events:
- Trip invitation (member added)
- Booking created (new reservation)
- Vacay fusion invite
- Photos shared (Immich)
- Collab chat message
- Packing list category assignment

User Notification Preferences:
- Per-user toggle for each event type in Settings
- Addon-aware: Vacay/Collab/Photos toggles hidden when addon disabled
- Webhook opt-in per user

ICS Calendar Export:
- Download button next to PDF in day plan header
- Exports trip dates + all reservations with details
- Compatible with Google Calendar, Apple Calendar, Outlook

Technical:
- Nodemailer for SMTP
- notification_preferences DB table with per-event columns
- GET/PUT /auth/app-settings for admin config persistence
- POST /notifications/test-smtp for validation
- Dynamic imports for non-blocking notification sends

server/src/db/migrations.ts

@@ -333,6 +333,29 @@ function runMigrations(db: Database.Database): void {
       // Add target_date to bucket_list for optional visit planning
       try { db.exec('ALTER TABLE bucket_list ADD COLUMN target_date TEXT DEFAULT NULL'); } catch {}
     },
+    () => {
+      // Notification preferences per user
+      db.exec(`CREATE TABLE IF NOT EXISTS notification_preferences (
+        id INTEGER PRIMARY KEY AUTOINCREMENT,
+        user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+        notify_trip_invite INTEGER DEFAULT 1,
+        notify_booking_change INTEGER DEFAULT 1,
+        notify_trip_reminder INTEGER DEFAULT 1,
+        notify_vacay_invite INTEGER DEFAULT 1,
+        notify_photos_shared INTEGER DEFAULT 1,
+        notify_collab_message INTEGER DEFAULT 1,
+        notify_packing_tagged INTEGER DEFAULT 1,
+        notify_webhook INTEGER DEFAULT 0,
+        UNIQUE(user_id)
+      )`);
+    },
+    () => {
+      // Add missing notification preference columns for existing tables
+      try { db.exec('ALTER TABLE notification_preferences ADD COLUMN notify_vacay_invite INTEGER DEFAULT 1'); } catch {}
+      try { db.exec('ALTER TABLE notification_preferences ADD COLUMN notify_photos_shared INTEGER DEFAULT 1'); } catch {}
+      try { db.exec('ALTER TABLE notification_preferences ADD COLUMN notify_collab_message INTEGER DEFAULT 1'); } catch {}
+      try { db.exec('ALTER TABLE notification_preferences ADD COLUMN notify_packing_tagged INTEGER DEFAULT 1'); } catch {}
+    },
   ];
 
   if (currentVersion < migrations.length) {