feat: health API — add build_timestamp + detailed resource status list

- build.rs: BUILD_TIMESTAMP from build time via `date -u`
- GET /health: now returns build_timestamp
- GET /health/detailed: returns build_timestamp + resources block
  (cpu_used/cpu_idle/memory/gpu usage)

build.rs

@@ -9,6 +9,15 @@ fn main() {
         .map(|s| s.trim().to_string())
         .unwrap_or_else(|| "unknown".to_string());
 
+    let timestamp = std::process::Command::new("date")
+        .args(["-u", "+%Y-%m-%dT%H:%M:%SZ"])
+        .output()
+        .ok()
+        .and_then(|o| String::from_utf8(o.stdout).ok())
+        .map(|s| s.trim().to_string())
+        .unwrap_or_else(|| "unknown".to_string());
+
     println!("cargo:rustc-env=BUILD_VERSION={}", version);
     println!("cargo:rustc-env=BUILD_GIT_HASH={}", git_hash);
+    println!("cargo:rustc-env=BUILD_TIMESTAMP={}", timestamp);
 }