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/database/db/schema.ts

@@ -2278,4 +2278,27 @@ export const pluginInstallCounts = sqliteTable(
   ],
 );
 
+/**
+ * Per-plugin key/value state, written through ctx.storage. Rows are always
+ * scoped to the calling plugin by the broker, so a plugin cannot name another
+ * plugin's scope. Cascades with the plugin so uninstalling leaves nothing.
+ */
+export const pluginStorage = sqliteTable(
+  "plugin_storage",
+  {
+    id: integer("id").primaryKey({ autoIncrement: true }),
+    pluginId: text("plugin_id")
+      .notNull()
+      .references(() => plugins.id, { onDelete: "cascade" }),
+    key: text("key").notNull(),
+    value: text("value").notNull(),
+    updatedAt: text("updated_at")
+      .notNull()
+      .default(sql`CURRENT_TIMESTAMP`),
+  },
+  (table) => [
+    uniqueIndex("idx_plugin_storage_plugin_key").on(table.pluginId, table.key),
+  ],
+);
+
 // --- plugins end ---