feat: objects table with M2M words/pictures/pairs

- objects: status enum, JSONB selections, notes, blocked_topic, auto-timestamps
- pairs placeholder table for future use
- Junction tables: object_words, object_pictures, object_pairs
- Full CRUD + link/unlink endpoints for all three relations
- README updated

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

src/db-migrate.js

@@ -116,6 +116,72 @@ async function migrate() {
     )
   `);
 
+  // pairs placeholder (Felder folgen später)
+  await query(`
+    CREATE TABLE IF NOT EXISTS pairs (
+      id         UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+      created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+      updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+    )
+  `);
+
+  await query(`
+    DROP TRIGGER IF EXISTS pairs_updated_at ON pairs;
+    CREATE TRIGGER pairs_updated_at
+      BEFORE UPDATE ON pairs
+      FOR EACH ROW EXECUTE FUNCTION update_updated_at()
+  `);
+
+  // objects
+  await query(`
+    CREATE TABLE IF NOT EXISTS objects (
+      id            UUID        PRIMARY KEY DEFAULT gen_random_uuid(),
+      status        VARCHAR(20) NOT NULL DEFAULT 'draft'
+                      CHECK (status IN ('draft', 'blocked', 'published')),
+      selections    JSONB,
+      notes         TEXT,
+      blocked_topic TEXT,
+      published_at  TIMESTAMPTZ,
+      blocked_at    TIMESTAMPTZ,
+      created_at    TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+      updated_at    TIMESTAMPTZ NOT NULL DEFAULT NOW()
+    )
+  `);
+
+  await query(`
+    DROP TRIGGER IF EXISTS objects_updated_at ON objects;
+    CREATE TRIGGER objects_updated_at
+      BEFORE UPDATE ON objects
+      FOR EACH ROW EXECUTE FUNCTION update_updated_at()
+  `);
+
+  // M2M: objects <-> words
+  await query(`
+    CREATE TABLE IF NOT EXISTS object_words (
+      object_id UUID NOT NULL REFERENCES objects(id) ON DELETE CASCADE,
+      word_id   UUID NOT NULL REFERENCES words(id) ON DELETE CASCADE,
+      PRIMARY KEY (object_id, word_id)
+    )
+  `);
+
+  // M2M: objects <-> pictures
+  await query(`
+    CREATE TABLE IF NOT EXISTS object_pictures (
+      object_id  UUID NOT NULL REFERENCES objects(id) ON DELETE CASCADE,
+      picture_id UUID NOT NULL REFERENCES pictures(id) ON DELETE CASCADE,
+      PRIMARY KEY (object_id, picture_id)
+    )
+  `);
+
+  // M2M: objects <-> pairs (Platzhalter)
+  await query(`
+    CREATE TABLE IF NOT EXISTS object_pairs (
+      object_id UUID NOT NULL REFERENCES objects(id) ON DELETE CASCADE,
+      pair_id   UUID NOT NULL REFERENCES pairs(id) ON DELETE CASCADE,
+      PRIMARY KEY (object_id, pair_id)
+    )
+  `);
+
   console.log('Migration complete');
 }