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/storage.py

@@ -227,8 +227,14 @@ class RefreshTokenStorage:
         if not self._initialized:
             await self.initialize()
 
-        # Type narrowing: cipher is set after initialize()
-        assert self.cipher is not None
+        # ``assert`` is stripped under ``python -O``, which would silently
+        # turn a missing TOKEN_ENCRYPTION_KEY into an ``AttributeError`` on
+        # the next ``self.cipher.encrypt(...)``. Raise explicitly instead
+        # (PR #758 round-4 review medium 1).
+        if self.cipher is None:
+            raise RuntimeError(
+                "TOKEN_ENCRYPTION_KEY is not set — token storage operations unavailable"
+            )
         encrypted_token = self.cipher.encrypt(refresh_token.encode())
         now = int(time.time())
         scopes_json = json.dumps(scopes) if scopes else None
@@ -374,8 +380,14 @@ class RefreshTokenStorage:
         if not self._initialized:
             await self.initialize()
 
-        # Type narrowing: cipher is set after initialize()
-        assert self.cipher is not None
+        # ``assert`` is stripped under ``python -O``, which would silently
+        # turn a missing TOKEN_ENCRYPTION_KEY into an ``AttributeError`` on
+        # the next ``self.cipher.encrypt(...)``. Raise explicitly instead
+        # (PR #758 round-4 review medium 1).
+        if self.cipher is None:
+            raise RuntimeError(
+                "TOKEN_ENCRYPTION_KEY is not set — token storage operations unavailable"
+            )
 
         start_time = time.time()
         try:
@@ -461,8 +473,14 @@ class RefreshTokenStorage:
         if not self._initialized:
             await self.initialize()
 
-        # Type narrowing: cipher is set after initialize()
-        assert self.cipher is not None
+        # ``assert`` is stripped under ``python -O``, which would silently
+        # turn a missing TOKEN_ENCRYPTION_KEY into an ``AttributeError`` on
+        # the next ``self.cipher.encrypt(...)``. Raise explicitly instead
+        # (PR #758 round-4 review medium 1).
+        if self.cipher is None:
+            raise RuntimeError(
+                "TOKEN_ENCRYPTION_KEY is not set — token storage operations unavailable"
+            )
 
         async with aiosqlite.connect(self.db_path) as db:
             async with db.execute(
@@ -635,8 +653,14 @@ class RefreshTokenStorage:
         if not self._initialized:
             await self.initialize()
 
-        # Type narrowing: cipher is set after initialize()
-        assert self.cipher is not None
+        # ``assert`` is stripped under ``python -O``, which would silently
+        # turn a missing TOKEN_ENCRYPTION_KEY into an ``AttributeError`` on
+        # the next ``self.cipher.encrypt(...)``. Raise explicitly instead
+        # (PR #758 round-4 review medium 1).
+        if self.cipher is None:
+            raise RuntimeError(
+                "TOKEN_ENCRYPTION_KEY is not set — token storage operations unavailable"
+            )
 
         # Encrypt sensitive data
         encrypted_secret = self.cipher.encrypt(client_secret.encode())
@@ -708,8 +732,14 @@ class RefreshTokenStorage:
         if not self._initialized:
             await self.initialize()
 
-        # Type narrowing: cipher is set after initialize()
-        assert self.cipher is not None
+        # ``assert`` is stripped under ``python -O``, which would silently
+        # turn a missing TOKEN_ENCRYPTION_KEY into an ``AttributeError`` on
+        # the next ``self.cipher.encrypt(...)``. Raise explicitly instead
+        # (PR #758 round-4 review medium 1).
+        if self.cipher is None:
+            raise RuntimeError(
+                "TOKEN_ENCRYPTION_KEY is not set — token storage operations unavailable"
+            )
 
         async with aiosqlite.connect(self.db_path) as db:
             async with db.execute(