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

docs/source/resources/includes/plano_config_full_reference.yaml

@@ -178,6 +178,14 @@ overrides:
 routing:
   session_ttl_seconds: 600    # How long a pinned session lasts (default: 600s / 10 min)
   session_max_entries: 10000  # Max cached sessions before eviction (upper limit: 10000)
+  # session_cache controls the backend used to store affinity state.
+  # "memory" (default) is in-process and works for single-instance deployments.
+  # "redis" shares state across replicas — required for multi-replica / Kubernetes setups.
+  session_cache:
+    type: memory              # "memory" (default) or "redis"
+    # url is required when type is "redis". Supports redis:// and rediss:// (TLS).
+    # url: redis://localhost:6379
+    # tenant_header: x-org-id  # optional; when set, keys are scoped as plano:affinity:{tenant_id}:{session_id}
 
 # State storage for multi-turn conversation history
 state_storage:

docs/source/resources/includes/plano_config_full_reference_rendered.yaml

@@ -216,6 +216,8 @@ ratelimits:
     key: x-org-id
     value: acme-corp
 routing:
+  session_cache:
+    type: memory
   session_max_entries: 10000
   session_ttl_seconds: 600
 state_storage: