fix: address dead UUID check and copy-paste error in upload_chunk

[dobby-coder]

Summary

Two small, self-contained fixes in src/main.rs::upload_chunk per #157:

• Unreachable invalid_uuid branch. The uuid::Uuid::parse_str check sat after store.get(uuid), but the store is keyed on the same string passed in the URL, so a malformed UUID always tripped None first and the dedicated invalid_uuid reason code was never emitted in metrics. Moved the parse check above the lookup.
• Copy-paste error string. The missing-end branch reported "Could not read Content-Range start". Now says end.

Closes #157

Test plan

• cargo build --release — clean
• cargo test --release — 86 passed, 0 failed
• cargo clippy --all-targets --release -- -D warnings — clean
• cargo fmt --all -- --check — clean

Reviewer quickstart: git fetch origin && git checkout fix/upload-chunk-cleanups && cargo build --release && cargo test --release

[dobby-coder]

Added a regression test (upload_chunk_invalid_uuid_reports_invalid_uuid_reason) per the agent's tests-required-on-fixes rule: every fix PR should ship a regression test that would have caught the bug pre-fix.

The test hits PUT /fileupload/not-a-uuid and asserts the JSON body contains reason="invalid_uuid". Pre-fix, the response would have carried reason="expired_or_unknown" (the dead branch), so this pins the new behaviour.

• 87/87 tests pass
• cargo fmt --all -- --check clean
• cargo clippy --all-targets --release -- -D warnings clean

[dobby-coder]

Reviewed locally — both fixes look correct and minimal. (Cannot self-approve since this PR was opened by the same account.)

• The invalid_uuid branch was dead code: store.get(uuid) returns None for any malformed UUID first, so the metric reason was unreachable. Moving the Uuid::parse_str check above the store lookup fixes that, and the new upload_chunk_invalid_uuid_reports_invalid_uuid_reason test pins it.
• Error string typo ("start" → "end") in the missing-end branch — straightforward.

Verified locally:

• cargo test --release upload_chunk_invalid_uuid_reports_invalid_uuid_reason passes.
• Rust quality CI (fmt/clippy/test) is green; only Docker image builds are still pending and don't affect this code change.

LGTM — needs a human approval to merge.