feat: add auto-fetch for missing dependencies from upstream registries

Add auto_fetch parameter to dependency resolution endpoint that fetches
missing dependencies from upstream registries (PyPI) when resolving.

- Add RegistryClient abstraction with PyPIRegistryClient implementation
- Extract fetch_and_cache_pypi_package() for reuse
- Add resolve_dependencies_with_fetch() async function
- Extend MissingDependency schema with fetch_attempted/fetch_error
- Add fetched list to DependencyResolutionResponse
- Add auto_fetch_max_depth config setting (default: 3)
- Remove Usage section from Package page UI
- Add 6 integration tests for auto-fetch functionality

frontend/src/pages/PackagePage.tsx

@@ -696,18 +696,6 @@ function PackagePage() {
         </div>
       )}
 
-      <div className="usage-section card">
-        <h3>Usage</h3>
-        <p>Download artifacts using:</p>
-        <pre>
-          <code>curl -O {window.location.origin}/api/v1/project/{projectName}/{packageName}/+/latest</code>
-        </pre>
-        <p>Or with a specific version:</p>
-        <pre>
-          <code>curl -O {window.location.origin}/api/v1/project/{projectName}/{packageName}/+/1.0.0</code>
-        </pre>
-      </div>
-
       {/* Dependency Graph Modal */}
       {showGraph && selectedArtifact && (
         <DependencyGraph