feat: pairs table with questions/statements placeholders

- pairs: status, answer_type enum (yes_no/text/word), difficulty_level,
  FK to questions + 2x statements (positive/negative), auto-timestamps
- questions + statements placeholder tables for future use
- Safe ALTER TABLE migration for existing pairs placeholder
- /api/pairs CRUD route, answer_type required on create

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

README.md

@@ -152,12 +152,34 @@ words ──────────── word_pictures ───────
 
 **Relations:** M2M mit `words`, `pictures`, `pairs` (pairs folgt später)
 
-### Table: `pairs` (Platzhalter — Felder folgen)
+### Table: `pairs`
+
+| Column | Type | Default | Description |
+|---|---|---|---|
+| `id` | `UUID` | `gen_random_uuid()` | Primary key |
+| `status` | `VARCHAR(20)` | `draft` | `draft` · `blocked` · `published` |
+| `answer_type` | `VARCHAR(20)` | — | `yes_no` · `text` · `word` (required) |
+| `difficulty_level` | `SMALLINT` | `null` | 1–50 |
+| `blocked_topic` | `TEXT` | `null` | Grund/Thema der Blockierung |
+| `question_id` | `UUID` FK | `null` | → `questions.id` (SET NULL on delete) |
+| `positive_statement_id` | `UUID` FK | `null` | → `statements.id` (SET NULL on delete) |
+| `negative_statement_id` | `UUID` FK | `null` | → `statements.id` (SET NULL on delete) |
+| `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: `questions` (Platzhalter — Felder folgen)
 | Column | Type |
 |---|---|
 | `id` | `UUID` PK |
-| `created_at` | `TIMESTAMPTZ` |
-| `updated_at` | `TIMESTAMPTZ` |
+| `created_at` / `updated_at` | `TIMESTAMPTZ` |
+
+### Table: `statements` (Platzhalter — Felder folgen)
+| Column | Type |
+|---|---|
+| `id` | `UUID` PK |
+| `created_at` / `updated_at` | `TIMESTAMPTZ` |
 
 ### Table: `object_words` (Junction)
 | Column | Type |
@@ -387,6 +409,24 @@ curl -X PATCH "$BASE/api/objects/<ID>" \
 
 ---
 
+### Pairs
+
+#### `GET /api/pairs` — Liste (`?status=draft&answer_type=yes_no&limit=50`)
+#### `GET /api/pairs/:id` — Einzelnes Pair
+#### `POST /api/pairs` — Neues Pair anlegen (`answer_type` ist Pflichtfeld)
+
+```bash
+curl -X POST "$BASE/api/pairs" \
+  -H "Authorization: Bearer $TOKEN" \
+  -H "Content-Type: application/json" \
+  -d '{"answer_type": "yes_no", "difficulty_level": 15}'
+```
+
+#### `PATCH /api/pairs/:id` — Felder aktualisieren
+#### `DELETE /api/pairs/:id` — Pair löschen
+
+---
+
 ### Utilities
 
 #### `GET /api/tables`