Add email sync service for automated job application tracking

Background poller fetches emails via IMAP or Microsoft Graph API,
classifies them with Claude Haiku, and creates/updates JobApplication
records automatically. Includes manual sync endpoint and OAuth callback.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/services/database.go

@@ -41,6 +41,7 @@ func migrateDatabase(db *gorm.DB) error {
 		&models.JobApplication{},
 		&models.JobAppReference{},
 		&models.Bookmark{},
+		&models.ProcessedEmail{},
 	)
 	if err != nil {
 		return err