Redis-backed session cache for cross-replica model affinity (#879)

* add pluggable session cache with Redis backend

* add Redis session affinity demos (Docker Compose and Kubernetes)

* address PR review feedback on session cache

* document Redis session cache backend for model affinity

* sync rendered config reference with session_cache addition

* add tenant-scoped Redis session cache keys and remove dead log_affinity_hit

- Add tenant_header to SessionCacheConfig; when set, cache keys are scoped
  as plano:affinity:{tenant_id}:{session_id} for multi-tenant isolation
- Thread tenant_id through RouterService, routing_service, and llm handlers
- Use Cow<'_, str> in session_key to avoid allocation when no tenant is set
- Remove unused log_affinity_hit (logging was already inlined at call sites)

* remove session_affinity_redis and session_affinity_redis_k8s demos

config/plano_config_schema.yaml

@@ -441,6 +441,25 @@ properties:
         minimum: 1
         maximum: 10000
         description: Maximum number of session-pinned routing cache entries. Default 10000.
+      session_cache:
+        type: object
+        properties:
+          type:
+            type: string
+            enum:
+              - memory
+              - redis
+            default: memory
+            description: Session cache backend. "memory" (default) is in-process; "redis" is shared across replicas.
+          url:
+            type: string
+            description: Redis URL, e.g. redis://localhost:6379. Required when type is redis.
+          tenant_header:
+            type: string
+            description: >
+              Optional HTTP header name whose value is used as a tenant prefix in the cache key.
+              When set, keys are scoped as plano:affinity:{tenant_id}:{session_id}.
+        additionalProperties: false
     additionalProperties: false
   state_storage:
     type: object