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/config/config.go

@@ -29,6 +29,7 @@ var (
 
 type StorageConfig struct {
 	RootDirectory string
+	MaxRepos      int
 	Dedupe        bool
 	RemoteCache   bool
 	GC            bool
@@ -1144,6 +1145,17 @@ func (c *Config) IsRetentionEnabled() bool {
 	return c.isRetentionEnabledInternal()
 }
 
+func (c *Config) IsQuotaEnabled() bool {
+	if c == nil {
+		return false
+	}
+
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	return c.Storage.MaxRepos > 0
+}
+
 // IsCompatEnabled checks if compatibility mode is enabled.
 func (c *Config) IsCompatEnabled() bool {
 	if c == nil {