Evict remote objects from every cache layer (#128)

Evicting a remote object only deleted its Postgres row. The S3 index and the Redis TTL/ETag keys survived, so stale mirror metadata such as EPEL repodata kept being served.

- evict the artifact row, S3 index and Redis keys via `Engine.Evict`
- evict a directory with `<dir>/*`; other wildcards return 400, unknown remotes 404
- wait on the per-path fetch lock for single-path evicts; return 503 on timeout or a Redis error
- wildcard evicts take no lock; a fetch already in flight may re-cache its path
- return S3 list errors from `DeletePrefix` instead of deleting nothing

Reviewed-on: #128
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/server/server.go

@@ -196,8 +196,8 @@ func (s *Server) routes() chi.Router {
 
 		r.Route("/remotes/{name}/objects", func(r chi.Router) {
 			objHandler := v2.NewObjectsHandler(s.db)
-			r.Get("/", objHandler.Routes().ServeHTTP)
-			r.Delete("/*", objHandler.Routes().ServeHTTP)
+			r.Get("/", objHandler.Routes(s.engine).ServeHTTP)
+			r.Delete("/*", objHandler.Routes(s.engine).ServeHTTP)
 		})
 
 		r.Route("/locals/{name}/objects", func(r chi.Router) {