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/url.ts

@@ -0,0 +1,18 @@
+export const pgProtocols = ["postgres:", "postgresql:"];
+
+export function isPgUrl(url: string): boolean {
+  try {
+    const urlObj = new URL(url);
+    return pgProtocols.includes(urlObj.protocol);
+  } catch {
+    return false;
+  }
+}
+
+export function parsePgUrl(url: string): { connectionString: string; readonly: boolean } {
+  if (!isPgUrl(url)) throw new Error("Not a postgres: URL");
+  const urlObj = new URL(url);
+  const readonly = (urlObj.searchParams.get("readonly") ?? "0") !== "0";
+  urlObj.searchParams.delete("readonly");
+  return { connectionString: urlObj.toString(), readonly };
+}