Add Redis scan_keys iterator API.

Provide a SCAN-based key iterator for Redis adapters so callers can enumerate large keyspaces without relying on blocking KEYS lookups.

Co-authored-by: Cursor <[email protected]>

README.md

@@ -34,6 +34,18 @@ Requires Redis with the RedisJSON module (e.g. redis-stack).
 | -------------------- | ---------------------------------------------------- |
 | `REDIS_URI`          | Redis connection URL (e.g. `redis://localhost:6379`) |
 
+For key discovery:
+
+- `list_keys(pattern)` is simple and returns a `list[str]`, but it uses Redis `KEYS` and may block on large datasets.
+- `scan_keys(pattern, *, count=None)` is preferred for production use and yields keys incrementally via Redis `SCAN`.
+
+Example:
+
+```python
+for key in repo.scan_keys("user:*"):
+    print(key)
+```
+
 ### MinIO (`ObjectRepositoryInterface`)
 
 | Environment variable | Description                                 |