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.

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:**
-- `database_duplicate`: Non-unique value (Code 1062).
-- `database_constraint`: Foreign key violation (Codes 1451, 1452).
-- `database_schema`: Invalid column name (Codes 1054, 1146).
-- `validation`: Pydantic validation failed (Check `details` for field-specific errors).
+### HTTP Status Codes
+- **`400 Bad Request`**: Used for client-driven errors including invalid search fields, validation failures, and constraint violations.
+- **`403 Forbidden`**: Missing or invalid API Key / Account Context.
+- **`404 Not Found`**: Object ID does not exist.
+- **`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**
 
 ---