Add OIDC back-channel logout with session indexes

packages/kaya-session/README.md

@@ -66,6 +66,37 @@ session and creates a fresh empty one.
 Set `max_age=None` to disable server-side expiry (and the `Max-Age` cookie
 attribute) entirely.
 
+## Session indexes
+
+Authentication layers can attach reverse-index metadata to a session without
+adding it to the user-visible session dictionary:
+
+```python
+ctx.session.add_index('oidc:default:sub', 'user-123')
+```
+
+Index-capable stores persist these memberships on `save()` and maintain them
+across session ID rotation and deletion:
+
+```python
+session_ids = await store.find_sessions('oidc:default:sub', 'user-123')
+deleted = await store.delete_by_index('oidc:default:sub', 'user-123')
+```
+
+Use `session.remove_index(name, value)` to remove a membership while keeping
+the session. `SessionMixin.store` exposes the backing store for features such
+as back-channel logout.
+
+`InMemorySessionStore`, `RedisSessionStore`, and `MemcacheSessionStore` set
+`supports_indexes = True`. Custom stores can opt in by setting that flag,
+implementing `find_sessions()`, maintaining reverse memberships on `save()`,
+and removing them on `delete()`. `delete_by_index()` has a generic default
+implementation based on `find_sessions()` and `delete()`.
+
+Remote stores serialize an envelope containing both the session dictionary and
+its index metadata. Existing plain-dict payloads remain readable and are
+upgraded on the next save.
+
 ## Features
 
 - `Session`: dict-like session object with modification tracking
@@ -74,6 +105,7 @@ attribute) entirely.
 - `SessionMixin`: composable Kaya mixin managing session cookies and persistence
 - Session ID regeneration (`session.regenerate_id()`) and invalidation
   (`session.invalidate()`) for authentication layers
+- Reverse session indexes for looking up and invalidating sessions by metadata
 - WebSocket support: the session is exposed as `ws.session` in websocket
   handlers, loaded at connect time and persisted on close