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/Backend-Development.md

@@ -632,14 +632,16 @@ Every non-2xx HTTP response body is a JSON object with this structure:
   "detail": "Jail 'example' not found",
   "metadata": {
     "jail_name": "example"
-  }
+  },
+  "correlation_id": "550e8400-e29b-41d4-a716-446655440000"
 }
 ```
 
 **Fields:**
-- **`code`** (string, required): Machine-readable error code for client-side branching. Examples: `jail_not_found`, `rate_limit_exceeded`, `authentication_required`.
+- **`code`** (string, required): Machine-readable error code for client-side branching. Examples: `jail_not_found`, `rate_limit_exceeded`, `authentication_required`, `invalid_input`.
 - **`detail`** (string, required): Human-readable error message. Safe for displaying to users.
-- **`metadata`** (object, optional): Structured context data relevant to the error. Only includes data safe for client consumption (no sensitive internal state). Examples: offending parameter names, resource identifiers, time windows.
+- **`metadata`** (object, optional): Structured context data relevant to the error. Only includes data safe for client consumption (no sensitive internal state). Examples: offending parameter names, resource identifiers, field error counts, time windows.
+- **`correlation_id`** (string | null, optional): Unique request ID for tracing this error across logs and systems. Set by the `CorrelationIdMiddleware`. Use this to correlate client-side errors with server logs for debugging.
 
 ### Exception Hierarchy & Error Codes
 
@@ -655,6 +657,8 @@ All domain exceptions inherit from `DomainError` (defined in `backend/app/except
 | **401** | `AuthenticationError` | `authentication_required` | Authentication or authorization failure, invalid/expired credentials |
 | **429** | `RateLimitError` | `rate_limit_exceeded` | Rate limit exceeded, too many requests |
 
+**Note on request validation errors:** Pydantic validation errors (from request body type mismatches, missing required fields, etc.) are automatically caught by the `_request_validation_error_handler` and converted to `ErrorResponse` with `code="invalid_input"`. The `metadata` field includes `field_errors` (count of validation failures) and `first_field` (location of the first error field) to help clients debug malformed requests.
+
 ### Implementing Error Handlers
 
 Every exception category has a corresponding exception handler registered in `backend/app/main.py`. When a domain exception is raised:

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**