fix: add security checks and tests for code review

Security:
- Add authorization checks to list_packages, update_package, delete_package endpoints
- Add MAX_TOTAL_ARTIFACTS limit (1000) to prevent memory exhaustion during dependency resolution
- Add TooManyArtifactsError exception for proper error handling

UI:
- Display reverse dependency errors in PackagePage
- Add warning display for failed dependency fetches in DependencyGraph

Tests:
- Add unit tests for metadata extraction (deb, wheel, tarball, jar)
- Add unit tests for rate limit configuration
- Add unit tests for PyPI registry client

frontend/src/pages/PackagePage.tsx

@@ -78,7 +78,7 @@ function PackagePage() {
   // Reverse dependencies state
   const [reverseDeps, setReverseDeps] = useState<DependentInfo[]>([]);
   const [reverseDepsLoading, setReverseDepsLoading] = useState(false);
-  const [_reverseDepsError, setReverseDepsError] = useState<string | null>(null);
+  const [reverseDepsError, setReverseDepsError] = useState<string | null>(null);
   const [reverseDepsPage, setReverseDepsPage] = useState(1);
   const [reverseDepsTotal, setReverseDepsTotal] = useState(0);
   const [reverseDepsHasMore, setReverseDepsHasMore] = useState(false);
@@ -647,10 +647,13 @@ function PackagePage() {
         />
       )}
 
-      {/* Used By (Reverse Dependencies) Section - only show if there are reverse deps */}
-      {reverseDeps.length > 0 && (
+      {/* Used By (Reverse Dependencies) Section - only show if there are reverse deps or error */}
+      {(reverseDeps.length > 0 || reverseDepsError) && (
         <div className="used-by-section card">
           <h3>Used By</h3>
+          {reverseDepsError && (
+            <div className="error-message">{reverseDepsError}</div>
+          )}
           <div className="reverse-deps-list">
             <div className="deps-summary">
               {reverseDepsTotal} {reverseDepsTotal === 1 ? 'package depends' : 'packages depend'} on this: