style: enforce Rust formatting in backend

[ubugeeei]

Finding

backend fails cargo fmt --check. This means Rust formatting is not currently enforceable in CI.

Evidence

• cargo fmt --check in backend reports diffs in backend/src/ffmpeg/bin.rs, backend/src/ffmpeg/sw_decoder.rs, backend/src/ffmpeg.rs, and backend/src/main.rs.

Acceptance criteria

• Apply cargo fmt to backend.
• cargo fmt --check passes in backend.
• No behavioral changes.

[ubugeeei]

対応済みです。修正は PR #1 / commit 9cf4498 に反映済みです。