feat: add MOONLOCK_DEBUG env var for debug-level logging (v0.6.1)

Align with moongreet/moonset logging pattern — set MOONLOCK_DEBUG to
enable debug-level journal output for troubleshooting.

src/main.rs

@@ -196,7 +196,12 @@ fn setup_logging() {
             eprintln!("Failed to create journal logger: {e}");
         }
     }
-    log::set_max_level(log::LevelFilter::Info);
+    let level = if std::env::var("MOONLOCK_DEBUG").is_ok() {
+        log::LevelFilter::Debug
+    } else {
+        log::LevelFilter::Info
+    };
+    log::set_max_level(level);
 }
 
 fn install_panic_hook() {