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.
Filter versions from the compact index (/info/{name}) by fetching
timestamps from the versions API (/api/v1/versions/{name}.json).
Both requests run concurrently to minimize latency. If the versions
API is unavailable, the compact index is proxied unfiltered.
Handles platform-specific versions (e.g. 1.0.0-java) by matching
the compact index format.
All handler metadata and proxy requests were using http.DefaultClient directly,
bypassing any timeout or transport configuration. Added an HTTPClient field to
the Proxy struct with a 30-second default timeout, and updated every handler
to use it for upstream HTTP requests.
