feat: Implement trip invitation functionality and notification handling

- Added TripInvitationRepository for managing trip invitations.
- Created TripInvitation model with serialization methods.
- Implemented notification payload parser for handling FCM notifications.
- Enhanced NotificationService to manage trip invitations and related actions.
- Updated UserRepository to include user search functionality.
- Modified AuthRepository to store multiple FCM tokens.
- Added tests for trip invitation logic and notification payload parsing.
- Updated pubspec.yaml and pubspec.lock for dependency management.

functions/package-lock.json

@@ -7,13 +7,14 @@
       "name": "functions",
       "dependencies": {
         "firebase-admin": "^13.6.0",
-        "firebase-functions": "^7.0.0"
+        "firebase-functions": "^7.0.0",
+        "nodemailer": "^6.10.1"
       },
       "devDependencies": {
         "firebase-functions-test": "^3.4.1"
       },
       "engines": {
-        "node": "24"
+        "node": "20"
       }
     },
     "node_modules/@babel/code-frame": {
@@ -5209,6 +5210,15 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/nodemailer": {
+      "version": "6.10.1",
+      "resolved": "https://registry.npmjs.org/nodemailer/-/nodemailer-6.10.1.tgz",
+      "integrity": "sha512-Z+iLaBGVaSjbIzQ4pX6XV41HrooLsQ10ZWPUehGmuantvzWoDVBnmsdUcOIDM1t+yPor5pDhVlDESgOMEGxhHA==",
+      "license": "MIT-0",
+      "engines": {
+        "node": ">=6.0.0"
+      }
+    },
     "node_modules/normalize-path": {
       "version": "3.0.0",
       "resolved": "https://registry.npmjs.org/normalize-path/-/normalize-path-3.0.0.tgz",