refactor(handlers): migrate remaining backend handlers to clock.Now() and transaction patterns

Apply clock.Now() migration, transaction wrapping, and minor refactors across admin, scheduling, today, user, auth handler, notifications, webhooks, services, portfolio, ratelimit, testutils, and main.go.

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

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

backend/handlers/user/customer_relationship.go

@@ -126,13 +126,18 @@ func GetCustomerRelationshipHandler(w http.ResponseWriter, r *http.Request) {
 			}
 			result.TopServices = append(result.TopServices, ts)
 		}
+		if err := rows.Err(); err != nil {
+			log.Printf("Row iteration error: %v", err)
+			http.Error(w, "Internal server error", http.StatusInternalServerError)
+			return
+		}
 	}
 
 	if result.TopServices == nil {
 		result.TopServices = []TopService{}
 	}
 
-	w.Header().Set("Content-Type", "application/json")
+
 	w.WriteHeader(http.StatusOK)
 	if err := json.NewEncoder(w).Encode(result); err != nil {
 		log.Printf("Failed to encode customer relationship response: %v", err)