MCP create_request rejects lowercase priority values

The MCP's create_request tool passes priority straight to the database. The requests_priority_check constraint only allows 'Low', 'Medium', 'High' and 'Urgent' (capitalized).

Steps to reproduce: call create_request with priority "medium".

Result: HTTP 400 "new row for relation \"requests\" violates check constraint \"requests_priority_check\"".

Expected: the request is created with Medium priority.

Suggested fix:

- Normalize priority case-insensitively before inserting.

- Expose the allowed values as an enum in the tool schema. It's currently a free-form string.

- Return a friendly validation error that lists the valid values, not a raw constraint error.

- Apply the same fix to effort if it has a similar constraint.