Fix T10.2: install SessionMiddleware, required by Authlib

okta_login() and okta_callback() (T10.2) both crash with a 500
(AssertionError: SessionMiddleware must be installed to access
request.session) against a real Okta client, because Authlib's
authorize_redirect() and authorize_access_token() both store/read OIDC
state and nonce in request.session. Never caught by T10.2's or T10.3's
own verification because every prior test mocked authorize_redirect /
authorize_access_token directly, bypassing Authlib's real implementation
entirely. Found while starting T10.5 and reproducing the real flow.

Adds starlette.middleware.sessions.SessionMiddleware, on its own cookie
(wp_oauth_state, distinct from the app's real session cookie wp_session)
with a short 10-minute lifetime and same_site=lax so it survives the
top-level redirect back from Okta. This cookie carries nothing but
ephemeral per-attempt OAuth state — no identity, no long-term secret —
so it reuses auth.SECRET_KEY rather than adding a new required config
knob. Reused in T10.5 to carry the post-login redirect target across
the same round trip.

Adds itsdangerous to requirements.txt — SessionMiddleware's hard
dependency, not previously needed anywhere in this app.

Verified: reproduced the crash against server.app with a fake (network-
bypassed) Authlib client and no SessionMiddleware, confirmed the
AssertionError, then confirmed the same request succeeds (302 to the
authorize URL, wp_oauth_state cookie set) once the middleware is added.

wave-10.md T10.2 (bug fix)

server/app.py

@@ -24,6 +24,7 @@ from fastapi.staticfiles import StaticFiles
 from pydantic import BaseModel, ConfigDict, Field
 from sqlalchemy import select, delete, func
 from sqlalchemy.orm import Session
+from starlette.middleware.sessions import SessionMiddleware
 
 from .db import Base, engine, get_db
 from . import models, auth, notify, assets_db, okta_auth
@@ -58,6 +59,22 @@ if _origins:
         allow_methods=["*"], allow_headers=["*"], expose_headers=["X-Total-Count"],
     )
 
+# Authlib's Okta client needs request.session to carry the OIDC state/nonce (and,
+# below, our own post-login redirect target) across the round trip to Okta and
+# back — it raises an AssertionError without this. Bug found in T10.2 (those
+# routes never crashed in testing because every prior check mocked
+# authorize_redirect/authorize_access_token directly, bypassing Authlib's real
+# implementation); fixed here rather than reworking already-shipped T10.2 code.
+#
+# This is NOT the app's session cookie — wp_session (auth.py) still carries the
+# actual signed-in identity, unchanged. This cookie holds nothing but ephemeral,
+# per-attempt OAuth state, so it gets a short lifetime and a plain secret reuse
+# (auth.SECRET_KEY) rather than its own required config knob.
+app.add_middleware(
+    SessionMiddleware, secret_key=auth.SECRET_KEY, session_cookie="wp_oauth_state",
+    same_site="lax", https_only=False, max_age=600,
+)
+
 
 # ── Authentication gate ────────────────────────────────────────────────────────
 # Every /api/ data route requires a valid session cookie. Login, health, and the