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/2.auth/1.sessions.pg.sql

@@ -0,0 +1,7 @@
+CREATE TABLE "sessions" (
+    "uid" UUID NOT NULL REFERENCES "users"("uid") ON DELETE CASCADE,
+    "token" TEXT NOT NULL PRIMARY KEY,
+    "created_at" TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    "updated_at" TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    "expires_at" TIMESTAMPTZ NOT NULL DEFAULT NOW() + INTERVAL '7 days'
+);