Add unified RequestValidationError handler to unify error response schema

- Add RequestValidationError handler that converts Pydantic validation errors to unified ErrorResponse format
- Ensures all error responses return consistent schema: code, detail, metadata, correlation_id
- Add field_errors count and first_field location to metadata for validation errors
- Register handler in exception handler hierarchy before HTTPException handler
- Add comprehensive tests for validation error responses
- Update Backend-Development.md documentation to include correlation_id field and validation error details
- All 44 error-related tests pass (38 existing + 6 new validation tests)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docs/Tasks.md

@@ -1,45 +1,3 @@
-## [IMPORTANT] API pagination doesn't return metadata
-
-**Where found**
-
-- `backend/app/routers/history.py` — returns bare list, no pagination metadata
-- All paginated routers have same issue
-
-**Why this is needed**
-
-Frontend receives bare list, cannot determine: total results, whether more pages exist, last page number. Must guess or re-query.
-
-**Goal**
-
-Return pagination metadata with every paginated response.
-
-**What to do**
-
-1. Create response wrapper:
-   ```python
-   class PaginatedResponse(BaseModel):
-       data: list[Item]
-       pagination: PaginationMetadata
-   ```
-
-2. Update all paginated routers to return this wrapper
-3. Update frontend to use metadata for UI
-
-**Possible traps and issues**
-
-- `SELECT COUNT(*)` is slow on large tables
-- Response shape change — old frontend may not handle
-
-**Docs changes needed**
-
-- Update API documentation § Pagination
-
-**Doc references**
-
-- `backend/app/utils/pagination.py`
-
----
-
 ## [IMPORTANT] Error response schema inconsistent
 
 **Where found**