feat(api): standardize V3 error status codes and documentation

- Updated V3 CRUD routers to return 400 Bad Request for database schema
  errors (unknown columns) across all list and search endpoints.
- Fixed serialization issue in nested patch endpoint.
- Overhauled Section 7 of Frontend Integration Guide to document HTTP
  status code mappings for common error categories.

app/routers/api_crud_v3.py

@@ -262,6 +262,15 @@ async def get_obj_li(
             as_list=True,
         )
 
+    if sql_result is False:
+        # Standardized rich error bubbling
+        db_err = format_db_error(get_last_sql_error())
+        
+        # If it's a schema error (like Unknown Column), it's a 400 Bad Request
+        status_code = 400 if db_err.category == "database_schema" else 500
+        
+        return mk_resp(data=False, status_code=status_code, response=response, status_message="Listing failed due to database error.", details=db_err.dict())
+
     if sql_result:
         resp_data_li = [base_name(**record).dict(by_alias=serialization.by_alias, exclude_unset=serialization.exclude_unset, exclude_defaults=serialization.exclude_defaults, exclude_none=serialization.exclude_none) for record in sql_result]
         return mk_resp(data=resp_data_li, response=response)

app/routers/api_crud_v3_nested.py

@@ -310,7 +310,7 @@ async def patch_child_obj(
         return mk_resp(data=True, response=response, status_message="Updated successfully.")
     else:
         db_err = format_db_error(get_last_sql_error())
-        return mk_resp(data=False, status_code=400, response=response, status_message="Update failed.", details=db_err)
+        return mk_resp(data=False, status_code=400, response=response, status_message="Update failed.", details=db_err.dict())
 
 
 @router.delete('/{parent_obj_type}/{parent_obj_id}/{child_obj_type}/{child_obj_id}', response_model=Resp_Body_Base)

documentation/GUIDE__V3_FRONTEND_API.md

@@ -132,11 +132,18 @@ V3 uses a **String-Only ID Vision**. The frontend NEVER handles or stores databa
 
 V3 returns machine-readable error objects in `meta.details` for failures.
 
-**Common Categories:**
+### HTTP Status Codes
-- `database_duplicate`: Non-unique value (Code 1062).
+- **`400 Bad Request`**: Used for client-driven errors including invalid search fields, validation failures, and constraint violations.
-- `database_constraint`: Foreign key violation (Codes 1451, 1452).
+- **`403 Forbidden`**: Missing or invalid API Key / Account Context.
-- `database_schema`: Invalid column name (Codes 1054, 1146).
+- **`404 Not Found`**: Object ID does not exist.
-- `validation`: Pydantic validation failed (Check `details` for field-specific errors).
+- **`500 Internal Server Error`**: Unexpected server crash or database connection failure.
+
+### Common Error Categories
+Found in `meta.details.category`:
+- `database_duplicate`: Non-unique value (Code 1062). -> **400**
+- `database_constraint`: Foreign key violation (Codes 1451, 1452). -> **400**
+- `database_schema`: Invalid column name or missing field in the requested `view` (Codes 1054, 1146). -> **400**
+- `validation`: Pydantic validation failed (Check `details` for field-specific errors). -> **400**
 
 ---