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>

test/ports.json

@@ -454,5 +454,11 @@
       "begin": 11520,
       "end": 11529
     }
+  },
+  "blackbox/quota.bats": {
+    "zot": {
+      "begin": 11530,
+      "end": 11539
+    }
   }
 }