feat: add OIDC sign-in, roles, sessions and client address rule

Authorization-code flow with PKCE through an idp adapter (with a fake), ID
token validation, identity and roles resolved from the directory with a short
cache, refusal of unknown, disabled and locked accounts with one generic page,
sessions that end when the account becomes unavailable, sign-out that ends the
server-side session, role decorators and the trusted-proxy client address rule.
Adds the OIDC_* and PUBLIC_BASE_URL configuration.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N3t4WL7bd9bdHHNjEsxzqC

portal/adapters/idp/fake.py

@@ -0,0 +1,41 @@
+"""In-memory identity provider for tests (AD-1)."""
+
+from __future__ import annotations
+
+import secrets
+from collections.abc import Mapping
+
+from .base import Flow, Identity, IdpUnavailable, LoginRejected
+
+
+class FakeIdp:
+    """`begin` hands out a flow; `complete` accepts the one-time code from `callback_params`."""
+
+    def __init__(self) -> None:
+        self.unavailable = False
+        self.identity: Identity | None = Identity("alice", "sub-alice")
+        self._codes: dict[str, Identity | None] = {}
+
+    def sign_in_as(self, username: str | None) -> None:
+        self.identity = Identity(username, f"sub-{username}") if username else None
+
+    def begin(self) -> tuple[str, Flow]:
+        if self.unavailable:
+            raise IdpUnavailable("The identity provider is not available")
+        flow = Flow(secrets.token_urlsafe(8), secrets.token_urlsafe(8), secrets.token_urlsafe(8))
+        return f"https://idp.example.test/authorize?state={flow.state}", flow
+
+    def callback_params(self, flow: Flow) -> dict[str, str]:
+        code = secrets.token_urlsafe(8)
+        self._codes[code] = self.identity
+        return {"state": flow.state, "code": code}
+
+    def complete(self, params: Mapping[str, str], flow: Flow) -> Identity:
+        if self.unavailable:
+            raise IdpUnavailable("The identity provider is not available")
+        if "error" in params or params.get("state") != flow.state:
+            raise LoginRejected("The sign-in response does not match the request")
+        identity = self._codes.pop(params.get("code", ""), None)  # a code works once
+        if identity is None:
+            raise LoginRejected("The identity provider refused the code")
+        return identity