fix: add MANUAL_USER_MAPPINGS support for users with mismatched emails

JimKarvo's Gitea email (dimitris@jksoftware.gr) differs from Slack
email (jimkarvo@gmail.com), so the email-based Slack lookup fails.
Add MANUAL_USER_MAPPINGS env var (format: GiteaUsername:SlackID,...)
to bypass email lookup when a manual mapping exists.

Changes:
- cache.go: ParseManualMappings() parses env var format
- cache.go: CachedResolver checks manual mappings first (step 0)
- main.go: reads MANUAL_USER_MAPPINGS env var, passes to resolver

cmd/server/main.go

@@ -67,8 +67,15 @@ func main() {
 	slackClient := slacknotifier.New(&cfg.Notification.Slack, logger)
 	logger.Info().Msg("Slack notifier initialized")
 
-	// Initialize cached identity resolver
-	resolver := cache.NewCachedResolver(repo, emailLookup, slackClient, logger)
+	// Initialize cached identity resolver with optional manual mappings
+	manualMappings := cache.ParseManualMappings(os.Getenv("MANUAL_USER_MAPPINGS"))
+	if len(manualMappings) > 0 {
+		logger.Info().Int("count", len(manualMappings)).Msg("manual user mappings configured")
+		for username := range manualMappings {
+			logger.Debug().Str("username", username).Msg("manual mapping registered")
+		}
+	}
+	resolver := cache.NewCachedResolver(repo, emailLookup, slackClient, manualMappings, logger)
 	logger.Info().Msg("identity resolver initialized")
 
 	// Create processor adapter that implements webhook.EventHandler