refactor(api): split management.py into domain-focused modules

Split the monolithic management.py (1988 lines) into 4 focused modules:
- management.py: Server status, user sessions, shared helpers (~520 lines)
- passwords.py: App password provisioning for BasicAuth mode (~300 lines)
- webhooks.py: Webhook registration management (~290 lines)
- visualization.py: Search and PDF preview endpoints (~810 lines)

Backward compatibility maintained via __init__.py re-exports.
Updated test imports to use new module paths.

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

nextcloud_mcp_server/app.py

@@ -2112,7 +2112,7 @@ def get_app(transport: str = "streamable-http", enabled_apps: list[str] | None =
         settings.enable_multi_user_basic_auth and settings.enable_offline_access
     )
     if enable_management_apis:
-        from nextcloud_mcp_server.api.management import (
+        from nextcloud_mcp_server.api import (
             create_webhook,
             delete_app_password,
             delete_webhook,