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/server/oauth_tools.py

@@ -432,8 +432,10 @@ async def _check_logged_in(ctx: Context, user_id: str) -> str:
         # Store state in session for validation on callback
         storage = await get_shared_storage()
 
-        # Create OAuth session for Flow 2
-        session_id = f"flow2_{user_id}_{secrets.token_hex(8)}"
+        # Create OAuth session for Flow 2. Identifier is purely random
+        # so audit-log entries / DB rows don't carry the user_id in the
+        # session_id field (PR #758 round-4 review medium 3).
+        session_id = f"flow2_{secrets.token_hex(16)}"
         redirect_uri = f"{os.getenv('NEXTCLOUD_MCP_SERVER_URL', 'http://localhost:8000')}/oauth/callback"
 
         await storage.store_oauth_session(