Add household seeding & session (task 07)

- scripts/seed.ts: idempotent pnpm db:seed inserts "Home" household if none exists (powered by tsx)
- signIn callback: first member of household gets owner role, subsequent users get member
- src/lib/session.ts: getCurrentSession() returns { user, household, role }, throws if unauthenticated or unmembered
- /settings/household: member list for all roles; rename form gated to owner only (server action also enforces)

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

scripts/seed.ts

@@ -0,0 +1,22 @@
+import postgres from "postgres";
+import { drizzle } from "drizzle-orm/postgres-js";
+import { households } from "@/modules/_core/schema";
+
+const client = postgres(process.env["DATABASE_URL"]!);
+const db = drizzle(client);
+
+async function seed() {
+  const [existing] = await db.select().from(households).limit(1);
+  if (existing) {
+    console.log(`Household already exists ("${existing.name}"), skipping.`);
+  } else {
+    await db.insert(households).values({ name: "Home" });
+    console.log('Seeded household "Home".');
+  }
+  await client.end();
+}
+
+seed().catch((err) => {
+  console.error(err);
+  process.exit(1);
+});