feat(backend): migrate notification IDs from int to string and update cursor format

Change notification ID type from int to string (CHAR(12)) to match the schema migration.

- Update AdminNotification.ID field to string
- Change cursor format from int-based to string-based for pagination
- Use validators.IsValidID instead of strconv.Atoi for notification ID validation
- Update cursor format to use RFC3339Nano for precision

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/notifications/notifications.go

@@ -18,7 +18,7 @@ import (
 
 // Structs returned in JSON
 type AdminNotification struct {
-	ID               int        `json:"id"`
+	ID               string     `json:"id"`
 	Reason           string     `json:"reason"`
 	BookingID        *string    `json:"booking_id,omitempty"`
 	UserID           *string    `json:"user_id,omitempty"`
@@ -185,7 +185,7 @@ func GetNotifications(w http.ResponseWriter, r *http.Request) {
 	if len(notifications) > perPage {
 		notifications = notifications[:perPage]
 		last := notifications[len(notifications)-1]
-		cursor := last.CreatedAt.Format(time.RFC3339) + "|" + strconv.Itoa(last.ID)
+		cursor := last.CreatedAt.Format(time.RFC3339Nano) + "|" + last.ID
 		nextCursor = &cursor
 	}
 
@@ -227,12 +227,7 @@ func GetUnreadCount(w http.ResponseWriter, r *http.Request) {
 
 func AcknowledgeNotification(w http.ResponseWriter, r *http.Request) {
 	idStr := chi.URLParam(r, "id")
-	if idStr == "" {
+	if idStr == "" || !validators.IsValidID(idStr) {
-		http.Error(w, "missing notification ID", http.StatusBadRequest)
-		return
-	}
-	id, err := strconv.Atoi(idStr)
-	if err != nil || id <= 0 {
 		http.Error(w, "invalid notification ID", http.StatusBadRequest)
 		return
 	}
@@ -243,9 +238,9 @@ func AcknowledgeNotification(w http.ResponseWriter, r *http.Request) {
 		WHERE id = $1 AND acknowledged_at IS NULL
 	`
 
-	cmdTag, err := db.DB.Exec(r.Context(), query, id)
+	cmdTag, err := db.DB.Exec(r.Context(), query, idStr)
 	if err != nil {
-		log.Printf("Failed to acknowledge notification %d: %v", id, err)
+		log.Printf("Failed to acknowledge notification %s: %v", idStr, err)
 		http.Error(w, "Internal server error", http.StatusInternalServerError)
 		return
 	}