feat: categories table with full CRUD

- Fills out placeholder categories table with all fields
- Trilingual titles, status enum, difficulty level, auto-timestamps
- ALTER TABLE IF NOT EXISTS for safe migration on existing table
- /api/categories CRUD route, word_ids included in responses

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

README.md

@@ -120,12 +120,21 @@ words ──────────── word_pictures ───────
 | `word_id` | `UUID` FK → `words.id` CASCADE |
 | `picture_id` | `UUID` FK → `pictures.id` CASCADE |
 
-### Table: `categories` (Platzhalter — Felder folgen)
-| Column | Type |
-|---|---|
-| `id` | `UUID` PK |
-| `created_at` | `TIMESTAMPTZ` |
-| `updated_at` | `TIMESTAMPTZ` |
+### Table: `categories`
+
+| Column | Type | Default | Description |
+|---|---|---|---|
+| `id` | `UUID` | `gen_random_uuid()` | Primary key |
+| `titel_de` | `TEXT` | `null` | German title |
+| `titel_en` | `TEXT` | `null` | English title |
+| `titel_se` | `TEXT` | `null` | Swedish title |
+| `status` | `VARCHAR(20)` | `requested` | `requested` · `blocked` · `published` |
+| `difficulty_level` | `SMALLINT` | `null` | 1–50 |
+| `requested_at` | `TIMESTAMPTZ` | `NOW()` on insert | Auto-set on create |
+| `published_at` | `TIMESTAMPTZ` | `null` | Auto-set when status → `published` |
+| `blocked_at` | `TIMESTAMPTZ` | `null` | Auto-set when status → `blocked` |
+| `created_at` | `TIMESTAMPTZ` | `NOW()` | Auto-set on insert |
+| `updated_at` | `TIMESTAMPTZ` | `NOW()` | Auto-updated on every change (trigger) |
 
 ### Table: `word_categories` (Junction)
 | Column | Type |
@@ -279,6 +288,24 @@ curl -X PATCH "$BASE/api/words/<ID>" \
 
 ---
 
+### Categories
+
+#### `GET /api/categories` — Liste (`?status=requested&limit=50&offset=0`)
+#### `GET /api/categories/:id` — Einzelne Kategorie inkl. `word_ids`
+#### `POST /api/categories` — Neue Kategorie anlegen
+
+```bash
+curl -X POST "$BASE/api/categories" \
+  -H "Authorization: Bearer $TOKEN" \
+  -H "Content-Type: application/json" \
+  -d '{"titel_de": "Tiere", "titel_en": "Animals", "titel_se": "Djur", "difficulty_level": 10}'
+```
+
+#### `PATCH /api/categories/:id` — Felder aktualisieren (Status setzt Timestamp automatisch)
+#### `DELETE /api/categories/:id` — Kategorie löschen (entfernt auch alle Verknüpfungen)
+
+---
+
 ### Utilities
 
 #### `GET /api/tables`