Close temporary clip upload files on every server error path

[Luqueee]

Problem

In internal/clip/server.go:103-132, the temporary upload file is not closed before returning a 413 MaxBytesError.

Repeated oversized uploads can exhaust file descriptors over the lifetime of the clip server.

Acceptance criteria

• Close the temporary file on every path, including 413, copy failure, close failure, and rename failure.
• Keep cleanup idempotent and avoid double-close assumptions.
• Preserve atomic publication and temporary-file removal.
• Add a regression test that exercises repeated oversized uploads and verifies no descriptor growth or stale open files.