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/src/tavolo/platform/mixin.py

@@ -2,7 +2,7 @@
 
 ``Platform`` bundles the collaborators every platform endpoint needs —
 the game registry, the live-game store, the deadline scheduler and the
-OIDC mixin used for authentication — so route and websocket modules
+OIDC providers used for authentication — so route and websocket modules
 never reach for module-global state. :class:`PlatformMixin` is a plain
 :class:`~kaya.core.KayaMixin`: applying it registers the lobby, stats
 and health HTTP routes plus the live-play websocket endpoint, and the
@@ -14,7 +14,7 @@ from __future__ import annotations
 from dataclasses import dataclass
 
 from kaya.core import KayaApp, KayaMixin
-from kaya.oidc import OIDCMixin
+from kaya.oidc import OIDCProviders
 
 from .deadlines import DeadlineScheduler
 from .registry import GameRegistry
@@ -28,7 +28,7 @@ class Platform:
     registry: GameRegistry
     game_store: GameStore
     scheduler: DeadlineScheduler
-    oidc: OIDCMixin
+    oidc: OIDCProviders
 
 
 class PlatformMixin(KayaMixin):
@@ -41,9 +41,10 @@ class PlatformMixin(KayaMixin):
         # Imported here so module import order stays acyclic: the route
         # modules reference Platform only for typing.
         from . import ws
-        from .routes import games, health, me, stats
+        from .routes import auth, games, health, me, stats
 
         health.register(app, self.platform)
+        auth.register(app, self.platform)
         me.register(app, self.platform)
         games.register(app, self.platform)
         stats.register(app, self.platform)