Make serveReaderFast ETag-aware (low priority)

Follow-up from the ETag support assessment (label: etag-range-followup). Low priority / optimisation, not correctness.

`internal/strategy/git/snapshot.go:serveReaderFast` calls `http.ServeContent` with a zero `ModTime` and empty name, and only for `*os.File` readers, which neutralises the built-in conditional and range handling in `ServeContent`.

### Work
Set the ETag (and use it consistently) so `ServeContent` can perform `304`/range responses natively for file-backed snapshots.

### Acceptance
File-backed snapshot serving returns `304` for a matching `If-None-Match` and honours `Range` via `ServeContent`.