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/migrations/3.notes/index.ts

@@ -0,0 +1,8 @@
+import type { PgMigration } from "../types.js";
+import p1 from "./1.notes.pg.sql";
+
+export const m3: PgMigration = {
+  id: 3,
+  name: "notes",
+  parts: [{ id: 1, name: "notes", sql: p1 }],
+};