feat: add plugin worker protocol, bootstrap and plugin_storage table

Worker-side ctx proxy plus the message envelope it talks over, and the
per-plugin key/value table behind ctx.storage with migrations for all
three dialects. Rows cascade with the plugin so uninstalling leaves
nothing behind.

src/backend/plugins/paths.ts

@@ -0,0 +1,22 @@
+import path from "node:path";
+
+/**
+ * Where unpacked plugins live. Resolved per call rather than captured at module
+ * scope: tests set DATA_DIR in beforeEach, and a module-scope constant would
+ * freeze whatever the first import saw.
+ */
+export function getPluginsDir(): string {
+  return path.join(process.env.DATA_DIR || "./db/data", "plugins");
+}
+
+export function getPluginDir(pluginId: string): string {
+  return path.join(getPluginsDir(), pluginId);
+}
+
+export function getPluginBackendEntry(pluginDir: string): string {
+  return path.join(pluginDir, "backend", "index.mjs");
+}
+
+export function getPluginManifestPath(pluginDir: string): string {
+  return path.join(pluginDir, "manifest.json");
+}