Implement scheduled email reminders

- Add cron service to Docker container for automated email sending
- Create /api/send-scheduled-emails endpoint for cron job calls
- Modify prayer submission to store user reminder preferences instead of sending immediate emails
- Update Firestore rules to allow server-side API access for scheduled emails
- Add userId to prayers for proper user association
- Cron runs every 5 minutes to check and send scheduled reminders

firestore.rules

@@ -11,6 +11,16 @@ service cloud.firestore {
       allow read, write: if request.auth != null;
     }
 
+    // Allow server-side API access for scheduled emails (no auth required for GET)
+    match /prayers {
+      allow read: if true;
+    }
+
+    // Allow server-side API access for scheduled emails
+    match /users {
+      allow read: if true;
+    }
+
     // Allow authenticated users to read/write config (for admin settings)
     match /config/{configId} {
       allow read, write: if request.auth != null;