feat(quota): support per‑client and anonymous server‑side request quotas

Unrestricted API usage can lead to runaway costs and fragmented client-side
throttling logic. This commit introduces a built-in quota mechanism at the
server level, enabling operators to centrally enforce per-client and anonymous
rate limits—without needing external proxies or client changes.

This helps contain compute costs, enforces fair usage, and simplifies deployment
and monitoring of Llama Stack services. Quotas are fully opt-in and have no
effect unless explicitly configured.

Currently, SQLite is the only supported KV store. If quotas are
configured but authentication is disabled, authenticated limits will
gracefully fall back to anonymous limits.

Highlights:
- Adds `QuotaMiddleware` to enforce request quotas:
  - Uses bearer token as client ID if present; otherwise falls back to IP address
  - Tracks requests in KV store with per-key TTL expiration
  - Returns HTTP 429 if a client exceeds their quota

- Extends `ServerConfig` with a `quota` section:
  - `kvstore`: configuration for the backend (currently only SQLite)
  - `anonymous_max_requests`: per-period cap for unauthenticated clients
  - `authenticated_max_requests`: per-period cap for authenticated clients
  - `period`: duration of the quota window (currently only `day` is supported)

- Adds full test coverage with FastAPI `TestClient` and custom middleware injection

Behavior changes:
- Quotas are disabled by default unless explicitly configured
- Anonymous users get a conservative default quota; authenticated clients can be given more generous limits

To enable per-client request quotas in `run.yaml`, add:
```yaml
server:
  port: 8321
  auth:
    provider_type: custom
    config:
      endpoint: https://auth.example.com/validate
  quota:
    kvstore:
      type: sqlite
      db_path: ./quotas.db
    anonymous_max_requests: 100
    authenticated_max_requests: 1000
    period: day
```

Signed-off-by: Wen Liang <wenliang@redhat.com>

docs/source/distributions/configuration.md

@@ -208,6 +208,80 @@ And must respond with:
 
 If no access attributes are returned, the token is used as a namespace.
 
+### Quota Configuration
+
+The `quota` section allows you to enable server-side request throttling for both
+authenticated and anonymous clients. This is useful for preventing abuse, enforcing
+fairness across tenants, and controlling infrastructure costs without requiring
+client-side rate limiting or external proxies.
+
+Quotas are disabled by default. When enabled, each client is tracked using either:
+
+* Their authenticated `client_id` (derived from the Bearer token), or
+* Their IP address (fallback for anonymous requests)
+
+Quota state is stored in a SQLite-backed key-value store, and rate limits are applied
+within a configurable time window (currently only `day` is supported).
+
+#### Example
+
+```yaml
+server:
+  quota:
+    kvstore:
+      type: sqlite
+      db_path: ./quotas.db
+    anonymous_max_requests: 100
+    authenticated_max_requests: 1000
+    period: day
+```
+
+#### Configuration Options
+
+| Field                        | Description                                                                |
+| ---------------------------- | -------------------------------------------------------------------------- |
+| `kvstore`                    | Required. Backend storage config for tracking request counts.              |
+| `kvstore.type`               | Must be `"sqlite"` for now. Other backends may be supported in the future. |
+| `kvstore.db_path`            | File path to the SQLite database.                                          |
+| `anonymous_max_requests`     | Max requests per period for unauthenticated clients.                       |
+| `authenticated_max_requests` | Max requests per period for authenticated clients.                         |
+| `period`                     | Time window for quota enforcement. Only `"day"` is supported.              |
+
+> Note: if `authenticated_max_requests` is set but no authentication provider is
+configured, the server will fall back to applying `anonymous_max_requests` to all
+clients.
+
+#### Example with Authentication Enabled
+
+```yaml
+server:
+  port: 8321
+  auth:
+    provider_type: custom
+    config:
+      endpoint: https://auth.example.com/validate
+  quota:
+    kvstore:
+      type: sqlite
+      db_path: ./quotas.db
+    anonymous_max_requests: 100
+    authenticated_max_requests: 1000
+    period: day
+```
+
+If a client exceeds their limit, the server responds with:
+
+```http
+HTTP/1.1 429 Too Many Requests
+Content-Type: application/json
+
+{
+  "error": {
+    "message": "Quota exceeded"
+  }
+}
+```
+
 ## Extending to handle Safety
 
 Configuring Safety can be a little involved so it is instructive to go through an example.