chore(): switched all fields in contract to snake_case and linting improvements

src/backend/api/v1/schemas.py

@@ -12,18 +12,18 @@ class FieldError(Schema):
     )
     issue: str = Field(..., description="Problem description")
     rejected_value: Any = Field(
-        None, alias="rejectedValue", description="Value that failed validation"
+        None, description="Value that failed validation"
     )
 
 
 class ApiError(Schema):
     code: str
     message: str
-    trace_id: str = Field(..., alias="traceId")
+    trace_id: str
     timestamp: datetime
     path: str
     details: dict[str, Any] | None = None
 
 
 class ValidationError(ApiError):
-    field_errors: list[FieldError] = Field(..., alias="fieldErrors")
+    field_errors: list[FieldError]