chore(deps): bump axum to 0.8

[haribo]

Summary

Supersedes #6. axum 0.8 migration in the relay:

• Message::Text now takes Utf8Bytes — .into() in the json() WS helper.
• Path params use {id} instead of :id (axum 0.8 panics at startup on the old syntax) — updated the /v1/devices/{id} and /v1/attachments/{id} routes. Handlers/Path extractors unchanged.

Test plan

• cargo test --workspace — all suites green, incl. the relay HTTP/WS integration tests (which start the server and hit the param routes)
• cargo clippy --workspace --all-targets -- -D warnings, cargo fmt --all --check

Closes #6