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/protocol.ts

@@ -0,0 +1,64 @@
+/**
+ * Wire format between a plugin worker and the main-thread broker.
+ *
+ * Everything crossing this boundary must be structured-cloneable. That is not
+ * just a serialisation detail: it is the reason a plugin cannot be handed a db
+ * handle, an ssh2 Client or a live socket by accident. Live objects stay in the
+ * main thread and the worker refers to them by opaque handle strings.
+ */
+
+export interface PluginRequest {
+  /** Correlates a reply to its request. Worker-generated, monotonic. */
+  id: number;
+  /** Dotted ctx path, e.g. "hosts.get" or "storage.set". */
+  method: string;
+  args: unknown[];
+}
+
+export interface PluginResponseOk {
+  id: number;
+  ok: true;
+  value: unknown;
+}
+
+export interface PluginResponseErr {
+  id: number;
+  ok: false;
+  error: { message: string; code?: string };
+}
+
+export type PluginResponse = PluginResponseOk | PluginResponseErr;
+
+/** Main thread -> worker, pushed rather than requested. */
+export interface PluginPush {
+  kind: "event" | "http" | "timer";
+  /** Event topic, route key, or timer id depending on kind. */
+  key: string;
+  payload: unknown;
+  /** Set when the push expects a reply (an HTTP route invocation). */
+  replyTo?: number;
+}
+
+/** First message the worker receives, before any plugin code runs. */
+export interface PluginBootstrapData {
+  pluginId: string;
+  pluginDir: string;
+  entryPath: string;
+  manifest: unknown;
+}
+
+export function isPluginRequest(value: unknown): value is PluginRequest {
+  if (!value || typeof value !== "object") return false;
+  const v = value as Record<string, unknown>;
+  return (
+    typeof v.id === "number" &&
+    typeof v.method === "string" &&
+    Array.isArray(v.args)
+  );
+}
+
+export function isPluginResponse(value: unknown): value is PluginResponse {
+  if (!value || typeof value !== "object") return false;
+  const v = value as Record<string, unknown>;
+  return typeof v.id === "number" && typeof v.ok === "boolean";
+}