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/token_utils.py

@@ -75,20 +75,31 @@ async def _get_cached(
     if entry is not None and time.time() < entry[0]:
         return entry[1]
     lock = await _get_fetch_lock(url)
-    async with lock:
-        # Re-check inside the lock — a concurrent waiter may have already
-        # populated the cache before we acquired it.
-        entry = cache.get(url)
-        if entry is not None and time.time() < entry[0]:
-            return entry[1]
-        async with nextcloud_httpx_client(
-            follow_redirects=follow_redirects
-        ) as http_client:
-            response = await http_client.get(url)
-            response.raise_for_status()
-            data = response.json()
-        cache[url] = (time.time() + _OIDC_CACHE_TTL, data)
-        return data
+    try:
+        async with lock:
+            # Re-check inside the lock — a concurrent waiter may have already
+            # populated the cache before we acquired it.
+            entry = cache.get(url)
+            if entry is not None and time.time() < entry[0]:
+                return entry[1]
+            async with nextcloud_httpx_client(
+                follow_redirects=follow_redirects
+            ) as http_client:
+                response = await http_client.get(url)
+                response.raise_for_status()
+                data = response.json()
+            cache[url] = (time.time() + _OIDC_CACHE_TTL, data)
+            return data
+    finally:
+        # Drop the dict entry so a misconfigured deployment hitting
+        # arbitrary URLs can't grow ``_fetch_locks`` without bound (PR #758
+        # round-4 review nit 4). Already-queued waiters share our local
+        # ``lock`` reference and remain coalesced; new arrivals lazily
+        # recreate a lock — by which time the cache is populated, so they
+        # short-circuit before reaching the lock anyway.
+        async with _fetch_locks_lock:
+            if _fetch_locks.get(url) is lock:
+                del _fetch_locks[url]
 
 
 async def get_oidc_discovery(discovery_url: str) -> dict[str, Any]: