Implement List Todos endpoint (GET /todos) with status filter

[landoyjx]

Implement GET /todos to return all Todo items. Support optional query parameter completed=true|false to filter results. Response should be 200 OK with an array of Todo objects.

Acceptance Criteria:

• Returns 200 with an array of all todos when no filter is provided
• Returns 200 with only completed todos when ?completed=true
• Returns 200 with only incomplete todos when ?completed=false
• Empty array is returned when no matching todos exist

Test Cases:

• Integration: GET /todos returns all todos in descending createdAt order (or consistent order)
• Integration: GET /todos?completed=true returns only todos where completed is true
• Integration: GET /todos?completed=false returns only todos where completed is false
• Edge: GET /todos?completed=invalid returns 400 with JSON error
• Edge: GET /todos when no todos exist returns 200 and []