Show click counts for short links

Users want to know whether their short links are being used.

What to build

• Every successful redirect through GET /{code} increments a click count for that code.
• GET /api/links/{code} returns the count as a new clicks field (0 for a new link). Existing fields are unchanged.
• Unknown codes keep returning 404 and are not counted.

Acceptance criteria

• Add a test in tests/test_app.py: create a link, follow it twice, and check that GET /api/links/{code} reports clicks == 2.
• All existing tests still pass.
• Keep the change small. Only app/main.py and tests/test_app.py should change.
• Open a merge request that says Closes this issue.