Bump axum from 0.7.9 to 0.8.4 in /backend (#55)

Bumps [axum](https://github.com/tokio-rs/axum) from 0.7.9 to 0.8.4.
- [Release notes](https://github.com/tokio-rs/axum/releases)
- [Changelog](https://github.com/tokio-rs/axum/blob/main/CHANGELOG.md)
- [Commits](tokio-rs/axum@axum-v0.7.9...axum-v0.8.4)

---
updated-dependencies:
- dependency-name: axum
  dependency-version: 0.8.4
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

backend/Cargo.lock

@@ -5,7 +5,7 @@
 
 [dependencies]
   async-trait = { version = "0.1" }
-  axum = { version = "0.7" }
+  axum = { version = "0.8" }
   bitcoin = { version = "0.32.5", features = [ "serde" ] }
   chrono = { version = "0.4.38", features = ["serde"] }
   clap = { version = "4.5", features = [ "derive" ] }