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>

examples/README.md

@@ -91,6 +91,16 @@ Orphan blobs are removed if they are older than gcDelay.
         "gcDelay": "2h"
 ```
 
+To limit the maximum number of repositories that can be created, set:
+
+```
+        "maxRepos": 10
+```
+
+When the limit is reached, pushes that would create a new repository are
+rejected with HTTP 429. Pushes to existing repositories are always allowed.
+Setting maxRepos to 0 or omitting it disables enforcement.
+
 It is also possible to store and serve images from multiple filesystems with
 their own repository paths, dedupe and garbage collection settings with: