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/server/oauth_tools.py

@@ -303,16 +303,17 @@ async def provision_nextcloud_access(
                 ),
             )
 
-        # Get configuration
-        enable_offline_access = (
-            os.getenv("ENABLE_OFFLINE_ACCESS", "false").lower() == "true"
-        )
-        if not enable_offline_access:
+        # Get configuration using settings (handles both ENABLE_BACKGROUND_OPERATIONS
+        # and ENABLE_OFFLINE_ACCESS environment variables)
+        from nextcloud_mcp_server.config import get_settings
+
+        settings = get_settings()
+        if not settings.enable_offline_access:
             return ProvisioningResult(
                 success=False,
                 message=(
                     "Offline access is not enabled. "
-                    "Set ENABLE_OFFLINE_ACCESS=true to use this feature."
+                    "Set ENABLE_BACKGROUND_OPERATIONS=true to use this feature."
                 ),
             )
 
@@ -488,13 +489,14 @@ async def check_logged_in(ctx: Context, user_id: Optional[str] = None) -> str:
         logger.info("=" * 60)
 
         # Not logged in - generate OAuth URL for Flow 2
-        enable_offline_access = (
-            os.getenv("ENABLE_OFFLINE_ACCESS", "false").lower() == "true"
-        )
-        if not enable_offline_access:
+        # Use settings (handles both ENABLE_BACKGROUND_OPERATIONS and ENABLE_OFFLINE_ACCESS)
+        from nextcloud_mcp_server.config import get_settings
+
+        settings = get_settings()
+        if not settings.enable_offline_access:
             return (
                 "Not logged in. Offline access is not enabled. "
-                "Set ENABLE_OFFLINE_ACCESS=true to use this feature."
+                "Set ENABLE_BACKGROUND_OPERATIONS=true to use this feature."
             )
 
         # Get MCP server's OAuth client credentials