fix(auth): address PR #758 review — XSS, CSRF, open redirect, JWKS cache

Addresses all 9 findings from the review on PR #758:

Blocking:
- _revoke_refresh_token_at_idp now reads config from oauth_ctx["config"]
  (the production-shaped nested dict). Previously read flat keys, causing
  IdP revocation to silently no-op in production. Test fixtures rebuilt
  to the realistic nested shape so the bug can't regress unnoticed.
- HTML error responses in oauth_login_callback now wrap IdP-controlled
  error_body, str(e), and the attacker-controlled error/error_description
  query params in html_escape. New test_browser_oauth_xss.py pins this.

Important:
- New _safe_next_url helper validates the ?next= query param at write
  time (oauth_login), in oauth_logout, and on read from the session row
  in oauth_login_callback. Blocks https://, // (protocol-relative), and
  CRLF/whitespace injection.
- verify_id_token now caches discovery + JWKS (5-min TTL) using the
  same pattern as oauth_routes._get_cached_discovery. New caching
  regression test pins to one fetch per URL across multiple calls.
- /oauth/logout is now POST-only at the route layer (defeats passive
  CSRF via <img src>). oauth_logout also validates Origin/Referer
  against the configured mcp_server_url. Logout UI in user_info.html
  converted from <a href> to <form method="post">.
- New storage.cleanup_expired_browser_sessions() called from the hourly
  cleanup loop in app.py — previously these rows accumulated for users
  who never explicitly logged out.

Nits:
- Demoted INFO logs that leaked oauth_config.keys() / client_id /
  token-storage state to DEBUG. Operator-relevant outcome lines
  (login successful, refresh token stored, logged out) stay INFO.
- verify_id_token algorithms widened to RS256, PS256, ES256 — covers
  Azure AD (PS256) and Cognito/some Keycloak realms (ES256). Symmetric
  and "none" remain off the allowlist.
- Migrated all Optional[X] usages in auth/storage.py to X | None per
  CLAUDE.md.

Breaking change: GET /oauth/logout now returns 405. The in-tree logout
UI was migrated to a POST form; any external bookmark or curl-based
caller that relied on GET will need to switch.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/test_browser_sessions.py

@@ -73,3 +73,27 @@ async def test_replace_existing_session_id(storage):
     await storage.create_browser_session(session_id=sid, user_id="bob")
 
     assert await storage.get_browser_session_user(sid) == "bob"
+
+
+async def test_cleanup_expired_browser_sessions(storage):
+    """Periodic cleanup removes expired rows but leaves fresh ones (PR #758 finding 6)."""
+    fresh_sid = secrets.token_urlsafe(32)
+    expired_sid = secrets.token_urlsafe(32)
+
+    await storage.create_browser_session(
+        session_id=fresh_sid, user_id="alice", ttl_seconds=3600
+    )
+    # ttl_seconds=-2 → expires_at strictly in the past (cleanup uses < now,
+    # so it must be actually less, not equal).
+    await storage.create_browser_session(
+        session_id=expired_sid, user_id="bob", ttl_seconds=-2
+    )
+
+    deleted = await storage.cleanup_expired_browser_sessions()
+    assert deleted == 1
+
+    # Fresh row survives, expired row is gone
+    assert await storage.get_browser_session_user(fresh_sid) == "alice"
+    assert await storage.get_browser_session_user(expired_sid) is None
+    # Calling again should be a no-op
+    assert await storage.cleanup_expired_browser_sessions() == 0