feat(api): add repository quota enforcement middleware (#3923)

Adds a configurable maximum repository count per registry instance.
When maxRepos is set on StorageConfig, manifest pushes that would create
a new repository beyond the limit are rejected with HTTP 429
TOOMANYREQUESTS. Pushes to existing repositories are always allowed.

Implemented as an always-available feature in pkg/api (not a build-tag
extension). MaxRepos is a field on StorageConfig, enabled when > 0.

- repoQuotaMiddleware on the dist-spec router intercepts manifest PUTs.
  New-repo pushes are serialized with a sync.Mutex to prevent concurrent
  requests from exceeding the limit.
- Adds CountRepos(ctx) to the MetaDB interface with efficient
  implementations: BoltDB (Stats().KeyN), Redis (HLen), DynamoDB
  (Scan with Select=COUNT).
- Config.IsQuotaEnabled() added, wired into controller.go metaDB init.
- Four integration tests (enforcement, concurrency, disabled,
  unconfigured) and backend-specific CountRepos tests for BoltDB, Redis,
  and DynamoDB.

Signed-off-by: Bachir Khiati <bachir.khiati@gmail.com>

pkg/api/routes.go

@@ -224,6 +224,7 @@ func (rh *RouteHandler) SetupRoutes() {
 	ext.SetupImageTrustRoutes(rh.c.Config, prefixedRouter, rh.c.MetaDB, rh.c.Log)
 	ext.SetupMgmtRoutes(rh.c.Config, prefixedRouter, rh.c.Log)
 	ext.SetupUserPreferencesRoutes(rh.c.Config, prefixedRouter, rh.c.MetaDB, rh.c.Log)
+	setupQuotaMiddleware(rh.c.Config, prefixedDistSpecRouter, rh.c.MetaDB, rh.c.Log)
 	// last should always be UI because it will setup a http.FileServer and paths will be resolved by this FileServer.
 	ext.SetupUIRoutes(rh.c.Config, rh.c.Router, rh.c.Log)
 }