feat: add PostgreSQL backend

Mirrors the node:sqlite sub-backend structure with full migration support.
Uses native pg types (UUID, JSONB, TIMESTAMPTZ) and $1/$2 parameterisation
via internal ? placeholders converted at execution time.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/db/postgres/getdb.static.ts

@@ -0,0 +1,20 @@
+import type { GetDbStatic } from "../types/GetDb.js";
+import { WrappedPool } from "./pool.js";
+import { PostgresInterface } from "./PostgresInterface.js";
+import { PostgresMigrator } from "./PostgresMigrator.js";
+import { isPgUrl, parsePgUrl, pgProtocols } from "./url.js";
+
+const getPgStatic: GetDbStatic = {
+  name: "postgres",
+  protocols: pgProtocols,
+  checkUrl: isPgUrl,
+  getDbInterface: async (url) => {
+    const { connectionString, readonly } = parsePgUrl(url);
+    return new PostgresInterface(new WrappedPool(connectionString, readonly));
+  },
+  getMigrator: async (url) => {
+    const { connectionString } = parsePgUrl(url);
+    return new PostgresMigrator(new WrappedPool(connectionString));
+  },
+};
+export default getPgStatic;