Add request_id Column to Error Log Table

## Motivation

The `error_logs` table currently lacks a `request_id` column, making it impossible to correlate errors with specific API requests and audit log entries. This prevents effective debugging and auditing of errors.

- Cannot link error logs to audit logs for the same request
- Cannot filter errors by request ID for debugging specific issues

## Required Features

- Create Alembic migration to add `request_id` column to `error_logs` table
- Column type: `String` (to match audit_logs.request_id)
- Add index on `request_id` for efficient querying
- Column should be nullable (existing records will have NULL)

## Impact

- `src/ai/backend/manager/models/error_logs.py`
- New Alembic migration file in `src/ai/backend/manager/models/alembic/versions/`

## Testing Scenarios

- Verify migration runs successfully (upgrade and downgrade)
- Verify new column is created with correct type and index
- Verify existing data is preserved after migration

JIRA Issue: BA-3724