style(backend): lowercase error messages across handlers

Normalize error message casing to lowercase for consistency across auth, bookings, services, customer relationship, and profile handlers.

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

@@ -36,7 +36,7 @@ type TopService struct {
 func GetCustomerRelationshipHandler(w http.ResponseWriter, r *http.Request) {
 	userID := chi.URLParam(r, "id")
 	if userID == "" || !validators.IsValidID(userID) {
-		http.Error(w, "User not found", http.StatusNotFound)
+		http.Error(w, "user not found", http.StatusNotFound)
 		return
 	}
 
@@ -52,7 +52,7 @@ func GetCustomerRelationshipHandler(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if !exists {
-		http.Error(w, "User not found", http.StatusNotFound)
+		http.Error(w, "user not found", http.StatusNotFound)
 		return
 	}
 

backend/handlers/user/profile.go

@@ -301,7 +301,7 @@ func UpdateProfileHandler(w http.ResponseWriter, r *http.Request) {
 func GetAdminUserHandler(w http.ResponseWriter, r *http.Request) {
 	userID := chi.URLParam(r, "id")
 	if userID == "" || !validators.IsValidID(userID) {
-		http.Error(w, "User not found", http.StatusNotFound)
+		http.Error(w, "user not found", http.StatusNotFound)
 		return
 	}
 
@@ -330,7 +330,7 @@ func GetAdminUserHandler(w http.ResponseWriter, r *http.Request) {
 
 	if err != nil {
 		if errors.Is(err, sql.ErrNoRows) {
-			http.Error(w, "User not found", http.StatusNotFound)
+			http.Error(w, "user not found", http.StatusNotFound)
 			return
 		}
 		log.Printf("Failed to fetch user %s: %v", userID, err)
@@ -610,7 +610,7 @@ type ServiceForPatchTest struct {
 func GetEligiblePatchTestServicesHandler(w http.ResponseWriter, r *http.Request) {
 	userID := chi.URLParam(r, "id")
 	if userID == "" || !validators.IsValidID(userID) {
-		http.Error(w, "User not found", http.StatusNotFound)
+		http.Error(w, "user not found", http.StatusNotFound)
 		return
 	}
 
@@ -665,7 +665,7 @@ type AddPatchTestRequest struct {
 func AddPatchTestHandler(w http.ResponseWriter, r *http.Request) {
 	userID := chi.URLParam(r, "id")
 	if userID == "" || !validators.IsValidID(userID) {
-		http.Error(w, "User not found", http.StatusNotFound)
+		http.Error(w, "user not found", http.StatusNotFound)
 		return
 	}
 
@@ -726,7 +726,7 @@ type UserPatchTest struct {
 func GetUserPatchTestsHandler(w http.ResponseWriter, r *http.Request) {
 	userID := chi.URLParam(r, "user_id")
 	if userID == "" || !validators.IsValidID(userID) {
-		http.Error(w, "User not found", http.StatusNotFound)
+		http.Error(w, "user not found", http.StatusNotFound)
 		return
 	}
 
@@ -763,7 +763,7 @@ func DeletePatchTestHandler(w http.ResponseWriter, r *http.Request) {
 	userID := chi.URLParam(r, "user_id")
 	testID := chi.URLParam(r, "test_id")
 	if userID == "" || !validators.IsValidID(userID) {
-		http.Error(w, "User not found", http.StatusNotFound)
+		http.Error(w, "user not found", http.StatusNotFound)
 		return
 	}
 	if testID == "" || !validators.IsValidID(testID) {