refactor: extract ProxyProvider interface with None and NPM implementations

Replace direct npm.Client usage throughout the codebase with the
proxy.Provider interface, enabling pluggable proxy backends. The
deployer, API layer, and proxy manager now use provider-agnostic
route management (ConfigureRoute/DeleteRoute) instead of NPM-specific
API calls. Adds ProxyRouteID (string) to Instance model and
ProxyProvider setting to Settings, with SQLite migrations for
backward compatibility.

internal/store/store.go

@@ -100,6 +100,10 @@ func (s *Store) runMigrations() error {
 		`ALTER TABLE settings ADD COLUMN backup_interval_hours INTEGER NOT NULL DEFAULT 24`,
 		`ALTER TABLE settings ADD COLUMN backup_retention_count INTEGER NOT NULL DEFAULT 10`,
 		`ALTER TABLE stages ADD COLUMN notification_url TEXT NOT NULL DEFAULT ''`,
+		// Add proxy_route_id to instances for provider-agnostic route tracking (2026-04-04).
+		`ALTER TABLE instances ADD COLUMN proxy_route_id TEXT NOT NULL DEFAULT ''`,
+		// Add proxy_provider to settings (2026-04-04). Default to npm for backward compat.
+		`ALTER TABLE settings ADD COLUMN proxy_provider TEXT NOT NULL DEFAULT 'npm'`,
 	}
 
 	for _, m := range migrations {