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/tests/helpers/oidc.py

@@ -1,8 +1,9 @@
 """Helpers for faking the OIDC authenticated user during tests.
 
-HTTP handlers funnel through ``oidc_mixin.get_user``; websocket handlers
-through :func:`tavolo.platform.auth.get_ws_user`. Patching those two entry
-points lets route and websocket tests run entirely in-process with no IdP.
+HTTP handlers funnel through ``OIDCProviders.get_users`` (wrapped by
+``tavolo.platform.auth.current_user``); websocket handlers through
+:func:`tavolo.platform.auth.get_ws_user`. Patching those two entry points
+lets route and websocket tests run entirely in-process with no IdP.
 """
 from __future__ import annotations
 
@@ -14,9 +15,13 @@ from kaya.oidc import OIDCUser
 
 # Import the app first: it pulls in the route modules, which import
 # ``tavolo.platform.auth`` themselves.
-from tavolo.app import oidc_mixin
+from tavolo.app import oidc
 from tavolo.platform import auth
 
+# The app's first configured provider (the test environment configures one
+# named "default").
+_PROVIDER = next(iter(oidc.providers))
+
 
 def make_user(sub: str, name: Optional[str] = None) -> OIDCUser:
     return OIDCUser({"sub": sub, "preferred_username": name or sub})
@@ -24,9 +29,9 @@ def make_user(sub: str, name: Optional[str] = None) -> OIDCUser:
 
 @contextlib.contextmanager
 def oidc_user(sub: str, name: Optional[str] = None) -> Iterator[OIDCUser]:
-    """Context manager: patch ``oidc_mixin.get_user`` to return this user."""
+    """Context manager: make ``oidc.get_users`` return this user."""
     user = make_user(sub, name)
-    patcher = _mock.patch.object(oidc_mixin, "get_user", return_value=user)
+    patcher = _mock.patch.object(oidc, "get_users", return_value={_PROVIDER: user})
     patcher.start()
     try:
         yield user