feat: add Docker image metadata support

- Store metadata (.meta.json) alongside manifests with:
  - push_timestamp, last_pulled, downloads counter
  - size_bytes, os, arch, variant
  - layers list with digest and size
- Update metadata on manifest pull (increment downloads, update last_pulled)
- Extract OS/arch from config blob on push
- Extend UI API TagInfo with metadata fields
- Add public_url config option for pull commands
- Add Docker upstream proxy with auth support
- Add raw repository support
- Bump version to 0.2.12

Cargo.lock

@@ -1185,7 +1185,7 @@ checksum = "38bf9645c8b145698bb0b18a4637dcacbc421ea49bef2317e4fd8065a387cf21"
 
 [[package]]
 name = "nora-cli"
-version = "0.2.11"
+version = "0.2.12"
 dependencies = [
  "clap",
  "flate2",
@@ -1199,7 +1199,7 @@ dependencies = [
 
 [[package]]
 name = "nora-registry"
-version = "0.2.11"
+version = "0.2.12"
 dependencies = [
  "async-trait",
  "axum",
@@ -1235,7 +1235,7 @@ dependencies = [
 
 [[package]]
 name = "nora-storage"
-version = "0.2.11"
+version = "0.2.12"
 dependencies = [
  "axum",
  "base64",