fix(auth): address PR #758 round-4 review

Seven findings from the latest review on #758 (3 medium, 4 low/nit):

Medium:
- storage.py: replace 5 ``assert self.cipher is not None`` sites with
  explicit ``RuntimeError`` so missing TOKEN_ENCRYPTION_KEY can't silently
  become an AttributeError under ``python -O``
- session_backend.py: document the silent-invalidation invariant —
  refresh-token TTL expiry without explicit logout deliberately makes
  the browser session unusable; future readers must not relax it
- server/oauth_tools.py: drop user_id from the Flow 2 session_id
  identifier — use ``flow2_{secrets.token_hex(16)}`` so audit logs and
  DB rows don't carry user_id in the session_id field

Low / nit:
- token_utils.py: drop _fetch_locks dict entry in finally so a probed
  deployment can't grow the lock dict without bound; coalescing test
  now pins the invariant with len(_fetch_locks) == 0
- browser_oauth_routes.py: strip trailing slash from settings.nextcloud_host
  before constructing the well-known URL so a host configured as
  ``https://cloud.example.com/`` doesn't produce a double-slash
- browser_oauth_routes.py: add comment explaining the three-layer CSRF
  policy on the mcp_session cookie set (SameSite=Lax + POST-only logout
  + Origin/Referer check)
- oauth_routes.py: convert all 23 f-string log calls to lazy %-style
  per the CLAUDE.md / memory feedback_lazy_logging convention

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

nextcloud_mcp_server/auth/browser_oauth_routes.py

@@ -614,6 +614,12 @@ async def oauth_login_callback(request: Request) -> RedirectResponse | HTMLRespo
     )
 
     response = RedirectResponse(next_url, status_code=302)
+    # CSRF protection is layered: ``SameSite=Lax`` blocks cross-site POSTs
+    # in modern browsers; ``oauth_logout`` is POST-only with an Origin /
+    # Referer check (``_origin_matches_self``) to cover older browsers and
+    # non-browser clients. ``HttpOnly`` blocks JS exfiltration on XSS;
+    # ``Secure`` is gated to non-HTTP hosts in dev (PR #758 round-4 review
+    # nit 6).
     response.set_cookie(
         key="mcp_session",
         value=session_id,
@@ -705,8 +711,12 @@ async def _revoke_refresh_token_at_idp(oauth_ctx: dict, refresh_token: str) -> N
     try:
         discovery_url = cfg.get("discovery_url") or settings.oidc_discovery_url
         if not discovery_url and settings.nextcloud_host:
+            # Strip trailing slash so a host configured as
+            # ``https://cloud.example.com/`` doesn't produce a double-slash
+            # in the well-known URL (PR #758 round-4 review nit 5).
             discovery_url = (
-                f"{settings.nextcloud_host}/.well-known/openid-configuration"
+                f"{settings.nextcloud_host.rstrip('/')}"
+                "/.well-known/openid-configuration"
             )
         if not discovery_url:
             return