fix: Add vendored OpenSSL for cross-compilation

- Add openssl dependency with vendored feature
- Fixes CI build for mips-unknown-linux-musl target
- Statically links OpenSSL instead of requiring system package
- Resolves openssl-sys build failures in cross-compile environment

backend/Cargo.toml

@@ -28,3 +28,4 @@ utoipa = { version = "5.4.0", features = ["axum_extras"] }
 utoipa-swagger-ui = { version = "9.0.2", features = ["axum"] }
 web-push = "0.10"
 base64 = "0.22"
+openssl = { version = "0.10", features = ["vendored"] }