Make clip GET metadata match the file that is actually served

[Luqueee]

Problem

In internal/clip/server.go:147-173, handleLatest stats the cache path before opening it.

An atomic replacement between Stat and Open can make Content-Length and Last-Modified describe a different inode from the bytes sent to the client.

Acceptance criteria

• Open the file first and obtain metadata from the opened file handle.
• Keep 404 behavior for a missing cache.
• Ensure headers and response body describe the same image.
• Add a concurrent PUT/GET regression test.