* Use shared artifacts at cache boundaries
* Address artifact cache review
* Defer cached artifact validation to checkCache
Construct artifacts.Artifact from the row without validation so a
malformed content hash reaches newIntegrityChecks in checkCache, which
clears the row and treats the request as a miss. Erroring at the DB
boundary instead surfaced a 500 and left the bad row in place.
* Build stored Artifact after digest and scan checks
Construct the shared artifact struct from trusted storage output as a
literal, after the hash-mismatch and scanner paths that delete failed
downloads, so no path between Store and updateCacheDB can leave bytes
in storage without a database row.
Add a `proxy mirror` CLI command and `/api/mirror` API endpoints that
pre-populate the cache from various input sources: individual PURLs,
SBOM files (CycloneDX and SPDX), or full registry enumeration.
The mirror reuses the existing handler.Proxy.GetOrFetchArtifact()
pipeline so cached artifacts are identical to those fetched on demand.
A bounded worker pool controls download parallelism.
Metadata caching is opt-in via `cache_metadata: true` in config (or
PROXY_CACHE_METADATA=true). The mirror command always enables it. When
enabled, upstream metadata responses are stored for offline fallback
with ETag-based conditional revalidation.
New internal/mirror package with Source interface, PURLSource,
SBOMSource, RegistrySource, and async JobStore. New metadata_cache
database table for offline metadata serving.
