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/init.pg.sql

@@ -0,0 +1,5 @@
+CREATE TABLE "_migrations" (
+    "id" INTEGER NOT NULL PRIMARY KEY,
+    "name" TEXT NOT NULL,
+    "applied_at" TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);