feat: add gc command and fix Docker-Content-Digest for Helm OCI

- Add nora gc --dry-run command for orphaned blob cleanup
- Fix Docker-Content-Digest header in blob upload response (enables Helm OCI push)
- Mark-and-sweep GC: list blobs, parse manifests, find/delete orphans

DevITWay

nora-registry/src/main.rs

@@ -11,6 +11,7 @@ mod health;
 mod metrics;
 mod migrate;
 mod openapi;
+mod gc;
 mod rate_limit;
 mod registry;
 mod repo_index;
@@ -61,6 +62,12 @@ enum Commands {
         #[arg(short, long)]
         input: PathBuf,
     },
+    /// Garbage collect orphaned blobs
+    Gc {
+        /// Dry run - show what would be deleted without deleting
+        #[arg(long, default_value = "false")]
+        dry_run: bool,
+    },
     /// Migrate artifacts between storage backends
     Migrate {
         /// Source storage: local or s3
@@ -143,6 +150,17 @@ async fn main() {
                 std::process::exit(1);
             }
         }
+        Some(Commands::Gc { dry_run }) => {
+            let result = gc::run_gc(&storage, dry_run).await;
+            println!("GC Summary:");
+            println!("  Total blobs:      {}", result.total_blobs);
+            println!("  Referenced:        {}", result.referenced_blobs);
+            println!("  Orphaned:          {}", result.orphaned_blobs);
+            println!("  Deleted:           {}", result.deleted_blobs);
+            if dry_run && !result.orphan_keys.is_empty() {
+                println!("\nRun without --dry-run to delete orphaned blobs.");
+            }
+        }
         Some(Commands::Migrate { from, to, dry_run }) => {
             let source = match from.as_str() {
                 "local" => Storage::new_local(&config.storage.path),