fix: Use settings.enable_offline_access for env var consolidation

Migrate all direct ENABLE_OFFLINE_ACCESS environment variable checks to
use settings.enable_offline_access, which handles both the new
ENABLE_BACKGROUND_OPERATIONS and deprecated ENABLE_OFFLINE_ACCESS vars.

Also fixes JWT issuer validation in Docker by using NEXTCLOUD_PUBLIC_ISSUER_URL
when set, resolving 401 errors caused by internal/external URL mismatch.

Changes:
- app.py: Use settings for offline access checks in setup_oauth_config,
  register_oauth_client, and tool registration
- oauth_tools.py: Use settings in provision_nextcloud_access and check_logged_in
- management.py: Use settings in get_user_session
- scope_authorization.py: Use settings in require_scopes decorator
- Remove unused os imports after migration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nextcloud_mcp_server/api/management.py

@@ -11,7 +11,6 @@ The PHP app obtains tokens through PKCE flow and uses them to access these endpo
 """
 
 import logging
-import os
 import time
 from importlib.metadata import version
 from typing import Any
@@ -347,11 +346,12 @@ async def get_user_session(request: Request) -> JSONResponse:
         )
 
     # Check if offline access is enabled
-    enable_offline_access = os.getenv("ENABLE_OFFLINE_ACCESS", "false").lower() in (
-        "true",
-        "1",
-        "yes",
-    )
+    # Use settings.enable_offline_access which handles both ENABLE_BACKGROUND_OPERATIONS (new)
+    # and ENABLE_OFFLINE_ACCESS (deprecated) environment variables
+    from nextcloud_mcp_server.config import get_settings
+
+    settings = get_settings()
+    enable_offline_access = settings.enable_offline_access
 
     if not enable_offline_access:
         # Offline access disabled - return minimal session info