### Task 4: Repository posts — autore e liste per autore

**Files:**
- Modify: `src/lib/posts.ts`
- Modify: `src/lib/auth.ts` (helper utenti riusati anche dal blog e dalla gestione utenti)
- Test: `tests/posts.test.ts`

**Interfaces:**
- Produces:
  - `interface Post` con `author_id: number | null`
  - `interface PostInput` con `author_id?: number | null`
  - `createPost(db, input)` scrive `author_id`
  - `listByAuthor(db, authorId: number): Post[]`
  - in `auth.ts`: `getUsernameById(db, id: number): string | null`, `listUsers(db): { id: number; username: string; role: string }[]`

- [ ] **Step 1: Scrivi i test del repository**

Aggiungi in `tests/posts.test.ts` (in coda, riusando `db`/`base`):

```typescript
import { listByAuthor } from '../src/lib/posts';
import { createUser, getUsernameById, listUsers } from '../src/lib/auth';

describe('autore articoli', () => {
  it('createPost salva author_id e listByAuthor filtra', () => {
    const aliceId = createUser(db, 'alice', 'segreta123', 'user');
    const bobId = createUser(db, 'bob', 'segreta123', 'user');
    createPost(db, { ...base, slug: 'a1', author_id: aliceId });
    createPost(db, { ...base, slug: 'a2', author_id: aliceId });
    createPost(db, { ...base, slug: 'b1', author_id: bobId });
    expect(listByAuthor(db, aliceId).length).toBe(2);
    expect(listByAuthor(db, bobId).length).toBe(1);
  });

  it('post senza author_id ha author_id null', () => {
    const id = createPost(db, { ...base, slug: 'orfano' });
    expect(getPostById(db, id)!.author_id).toBeNull();
  });

  it('getUsernameById e listUsers', () => {
    const id = createUser(db, 'carla', 'segreta123', 'superuser');
    expect(getUsernameById(db, id)).toBe('carla');
    expect(getUsernameById(db, 99999)).toBeNull();
    expect(listUsers(db).some((u) => u.username === 'carla' && u.role === 'superuser')).toBe(true);
  });
});
```

- [ ] **Step 2: Esegui e verifica il fallimento**

Run: `npm test -- tests/posts.test.ts`
Expected: FAIL — `listByAuthor`/`getUsernameById`/`listUsers` non esistono, `author_id` non nell'input.

- [ ] **Step 3: Aggiorna `posts.ts`**

Nell'`interface Post` aggiungi `author_id: number | null;`. Nell'`interface PostInput` aggiungi `author_id?: number | null;`. Sostituisci `createPost`:

```typescript
export function createPost(db: Database.Database, input: PostInput): number {
  const res = db.prepare(
    `INSERT INTO posts (title, slug, category, excerpt, body_html, cover, draft, author_id, published_at)
     VALUES (@title, @slug, @category, @excerpt, @body_html, @cover, @draft, @author_id,
             CASE WHEN @draft = 0 THEN datetime('now') ELSE NULL END)`
  ).run({ ...input, cover: input.cover ?? null, author_id: input.author_id ?? null, draft: input.draft ? 1 : 0 });
  return Number(res.lastInsertRowid);
}
```

`updatePost` resta invariato (non tocca `author_id`: l'autore non cambia in modifica). Aggiungi in fondo:

```typescript
export function listByAuthor(db: Database.Database, authorId: number): Post[] {
  return db.prepare('SELECT * FROM posts WHERE author_id = ? ORDER BY updated_at DESC').all(authorId) as Post[];
}
```

- [ ] **Step 4: Aggiungi gli helper utenti in `auth.ts`**

In `src/lib/auth.ts` aggiungi:

```typescript
export function getUsernameById(db: Database.Database, id: number): string | null {
  const row = db.prepare('SELECT username FROM users WHERE id = ?').get(id) as { username: string } | undefined;
  return row?.username ?? null;
}

export function listUsers(db: Database.Database): { id: number; username: string; role: string }[] {
  return db.prepare('SELECT id, username, role FROM users ORDER BY username').all() as
    { id: number; username: string; role: string }[];
}
```

- [ ] **Step 5: Esegui e verifica il successo**

Run: `npm test -- tests/posts.test.ts`
Expected: PASS.

- [ ] **Step 6: Commit**

```bash
git add src/lib/posts.ts src/lib/auth.ts tests/posts.test.ts
git commit -m "feat(posts): author_id nel repository + listByAuthor + helper utenti"
```

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