refactor: standardize error handling with exception.Builder

- Replace hardcoded error messages with structured error codes using exception.Builder.
- Introduce new common error constants in exception/common.go (CommonErrorInvalidInput, CommonErrorUserNotFound, etc.).
- Update exception/specific.go with domain-specific errors and remove redundant ones.
- Apply consistent error handling across auth, event, user services and middleware.

Co-authored-by: Gemini <gemini@google.com>
Signed-off-by: Noa Virellia <noa@requiem.garden>

exception/common.go

@@ -1,6 +1,12 @@
 package exception
 
 const (
-	CommonErrorInvalidInput = "00001"
-	CommonErrorUnauthorized = "00002"
+	CommonErrorInvalidInput     = "00001"
+	CommonErrorUnauthorized     = "00002"
+	CommonErrorInternal         = "00003"
+	CommonErrorPermissionDenied = "00004"
+	CommonErrorUuidParseFailed  = "00005"
+	CommonErrorDatabase         = "00006"
+	CommonErrorMissingUserId    = "00007"
+	CommonErrorUserNotFound     = "00008"
 )