feat(quota): add server‑side per‑client request quotas (requires auth) (#2096)

# What does this PR do?
feat(quota): add server‑side per‑client request quotas (requires auth)
    
Unrestricted usage can lead to runaway costs and fragmented client-side
    workarounds. This commit introduces a native quota mechanism to the
    server, giving operators a unified, centrally managed throttle for
    per-client requests—without needing extra proxies or custom client
logic. This helps contain cloud-compute expenses, enables fine-grained
usage control, and simplifies deployment and monitoring of Llama Stack
services. Quotas are fully opt-in and have no effect unless explicitly
    configured.
    
    Notice that Quotas are fully opt-in and require authentication to be
enabled. The 'sqlite' is the only supported quota `type` at this time,
any other `type` will be rejected. And the only supported `period` is
    'day'.
    
    Highlights:
    
    - Adds `QuotaMiddleware` to enforce per-client request quotas:
      - Uses `Authorization: Bearer <client_id>` (from
        AuthenticationMiddleware)
      - Tracks usage via a SQLite-based KV store
      - Returns 429 when the quota is exceeded
    
    - Extends `ServerConfig` with a `quota` section (type + config)
    
- Enforces strict coupling: quotas require authentication or the server
      will fail to start
    
    Behavior changes:
    - Quotas are disabled by default unless explicitly configured
    - SQLite defaults to `./quotas.db` if no DB path is set
    - The server requires authentication when quotas are enabled
    
    To enable per-client request quotas in `run.yaml`, add:
    ```
    server:
      port: 8321
      auth:
        provider_type: "custom"
        config:
          endpoint: "https://auth.example.com/validate"
      quota:
        type: sqlite
        config:
          db_path: ./quotas.db
          limit:
            max_requests: 1000
            period: day

[//]: # (If resolving an issue, uncomment and update the line below)
Closes #2093

## Test Plan
[Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.*]

[//]: # (## Documentation)

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