feat: add JWT/OAuth2 authentication & multi-tenant federation

- Add LLMAuth struct with support for Bearer, API Key, and Custom auth types
- Implement applyAuth() to inject auth headers into LLM requests
- Add X-Tenant-ID header for multi-tenant isolation
- Add X-OAuth-Scopes header for OAuth2 scope enforcement
- Add UpdateAuth() for runtime token refresh (long-running workflows)
- Update LLMRouterConfig with Auth and TenantID fields
- Document 4 authentication patterns (Bearer, API Key, Custom, Router config)
- Add security best practices: token vault integration, tenant isolation, scopes
- Add audit headers for compliance & logging
- Create multi-tenant router factory pattern

Auth types supported:
- Bearer: JWT/OAuth2 tokens (most secure for federated access)
- API Key: Static keys (X-API-Key header)
- Custom: Any custom header-based scheme
- None: No authentication

Customers can now pass per-tenant JWT tokens with customized scopes
and isolated LLM API access per tenant/customer.

k8s/worker-deployment.yaml

@@ -13,8 +13,8 @@ spec:
       labels:
         app: poimen-worker
       annotations:
-        git-commit: "f3ce019c"  # ✅ Updated on each push, triggers rolling restart
-        deployment-date: "2026-09-03"
+        git-commit: "30644a8e"  # ✅ Updated on each push, triggers rolling restart
+        deployment-date: "2026-09-04"
     spec:
       containers:
       - name: worker