Support multiple OIDC providers

kaya-oidc 0.0.6 can run several providers in one app (OIDCProviders),
each with its own login route and namespaced session state. Adopt it:
providers are configured in indexed, contiguous OIDC_<n>_* blocks
(0-based) with OIDC_<n>_PROVIDER naming the provider in its login URL
and session keys. The old unindexed OIDC_ISSUER/OIDC_CLIENT_ID/... names
are no longer read; startup fails with a rename hint when they are set.
With no OIDC variables at all, the single dev mock provider named
"default" is used, as before.

The platform now authenticates through OIDCProviders: auth.current_user
returns the session's user (sessions stay single-identity, so at most one
provider is logged in) and require_auth/route handlers use it, while the
websocket path keeps scanning for any provider's user. New routes expose
the configuration to the SPA and keep old links working: GET
/api/auth/providers lists the login URLs, GET /auth/login redirects to
the first provider and GET /auth/logout to whichever provider the
session is logged in with. The lobby renders one login button per
provider from that endpoint.

Update the env examples, compose and k8s manifests to the indexed scheme,
document the multiple-provider setup and the migration, and cover it with
config, auth and route tests.

server/packages/tavolo-platform/tests/helpers.py

@@ -8,7 +8,7 @@ without importing any real game. Its rules: the first player to reach
 ``target`` plays wins the match.
 
 :func:`make_platform` builds a throwaway :class:`~kaya.core.KayaApp`
-wired with in-memory stores, a dummy OIDC mixin (patched per test) and
+wired with in-memory stores, dummy OIDC providers (patched per test) and
 a sqlite :class:`~tavolo.platform.tortoise_mixin.TortoiseMixin`, so
 tests construct their own app instead of importing a global one.
 """
@@ -22,7 +22,7 @@ from functools import wraps
 from typing import Any, Callable, Coroutine, Dict, Iterator, List, Mapping, Optional, Sequence, Tuple
 
 from kaya.core import KayaApp
-from kaya.oidc import OIDCConfig, OIDCMixin, OIDCUser
+from kaya.oidc import OIDCConfig, OIDCProviders, OIDCUser
 from kaya.openapi import OpenAPIMixin
 from kaya.session import InMemorySessionStore, SessionMixin
 
@@ -221,20 +221,25 @@ class DummyEngine(GameEngine):
 def make_platform(
     engines: Sequence[GameEngine] = (DummyEngine(),),
     heartbeat_ms: int = 50,
+    oidc_providers: Sequence[str] = ("default",),
 ) -> Tuple[KayaApp, Platform, TortoiseMixin]:
     """Build a throwaway app + platform wired with in-memory stores."""
     registry = GameRegistry(engines)
     game_store = InMemoryGameStore(registry)
     session_mixin = SessionMixin(InMemorySessionStore())
-    oidc_mixin = OIDCMixin(
-        OIDCConfig(
-            issuer="http://localhost:8180/tavolo",
-            client_id="tavolo",
-            client_secret=None,
-            redirect_uri="http://localhost:8080/auth/callback",
-            post_login_redirect="/",
-            post_logout_redirect="/",
-        ),
+    oidc = OIDCProviders(
+        [
+            OIDCConfig(
+                provider=name,
+                issuer=f"http://localhost:8180/{name}",
+                client_id="tavolo",
+                client_secret=None,
+                redirect_uri=f"http://localhost:8080/auth/{name}/callback",
+                post_login_redirect="/",
+                post_logout_redirect="/",
+            )
+            for name in oidc_providers
+        ],
         session=session_mixin,
     )
     tortoise_mixin = TortoiseMixin(
@@ -259,12 +264,12 @@ def make_platform(
         registry=registry,
         game_store=game_store,
         scheduler=scheduler,
-        oidc=oidc_mixin,
+        oidc=oidc,
     )
     app = KayaApp(
         mixins=[
             session_mixin,
-            oidc_mixin,
+            oidc,
             tortoise_mixin,
             openapi_mixin,
             PlatformMixin(platform),
@@ -320,10 +325,12 @@ def make_user(sub: str, name: Optional[str] = None) -> OIDCUser:
 
 
 @contextlib.contextmanager
-def oidc_user(oidc: OIDCMixin, sub: str, name: Optional[str] = None) -> Iterator[OIDCUser]:
-    """Context manager: patch ``oidc.get_user`` to return this user."""
+def oidc_user(oidc: OIDCProviders, sub: str, name: Optional[str] = None) -> Iterator[OIDCUser]:
+    """Context manager: make ``oidc.get_users`` return this user for the
+    first configured provider."""
     user = make_user(sub, name)
-    patcher = _mock.patch.object(oidc, "get_user", return_value=user)
+    provider = next(iter(oidc.providers))
+    patcher = _mock.patch.object(oidc, "get_users", return_value={provider: user})
     patcher.start()
     try:
         yield user