fix: address PR #589 review feedback (round 2)

Consolidate three independent RefreshTokenStorage lazy singletons into a
single lock-protected get_shared_storage() function, eliminating race
conditions on concurrent first-access. Remove blanket try/except in
_get_stored_scopes so storage errors propagate as proper MCP errors
instead of silently triggering "please provision" messages. Handle
declined/cancelled elicitation results in Login Flow tools by cleaning up
sessions and returning clear status. Add update_app_password_scopes() to
avoid unnecessary decrypt/re-encrypt when only scopes change. Add
unprovisioned-user early exit and no-op detection to nc_auth_update_scopes.
Remove four dead config fields and misleading NEXTCLOUD_PASSWORD deprecation
warning. Add periodic login flow session cleanup task. Generate separate
Fernet keys per service. Add board cleanup in deck integration test. Gate
CI unit tests on linting and skip Astrolabe build for single-user profile.
Fix test markers from oauth to multi_user_basic for astrolabe integration
tests. Update login_flow.py docstrings to document outbound HTTP calls.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nextcloud_mcp_server/context.py

@@ -10,7 +10,7 @@ from nextcloud_mcp_server.auth.context_helper import (
     get_session_client_from_context,
 )
 from nextcloud_mcp_server.auth.scope_authorization import ProvisioningRequiredError
-from nextcloud_mcp_server.auth.storage import RefreshTokenStorage
+from nextcloud_mcp_server.auth.storage import get_shared_storage
 from nextcloud_mcp_server.client import NextcloudClient
 from nextcloud_mcp_server.config import (
     DeploymentMode,
@@ -254,18 +254,6 @@ def _get_client_from_basic_auth(ctx: Context) -> NextcloudClient:
     )
 
 
-_login_flow_storage_instance = None
-
-
-async def _get_login_flow_storage():
-    """Get initialized storage instance for login flow (lazy singleton)."""
-    global _login_flow_storage_instance
-    if _login_flow_storage_instance is None:
-        _login_flow_storage_instance = RefreshTokenStorage.from_env()
-        await _login_flow_storage_instance.initialize()
-    return _login_flow_storage_instance
-
-
 async def _get_client_from_login_flow(
     ctx: Context, nextcloud_host: str
 ) -> NextcloudClient:
@@ -294,7 +282,7 @@ async def _get_client_from_login_flow(
             "Cannot determine user identity from MCP token."
         )
 
-    storage = await _get_login_flow_storage()
+    storage = await get_shared_storage()
 
     app_data = await storage.get_app_password_with_scopes(user_id)
     if not app_data: