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>

src/db-migrate.js

@@ -68,15 +68,38 @@ async function migrate() {
     )
   `);
 
-  // categories (Platzhalter — Felder folgen im nächsten Schritt)
   await query(`
     CREATE TABLE IF NOT EXISTS categories (
-      id         UUID PRIMARY KEY DEFAULT gen_random_uuid(),
-      created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
-      updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+      id               UUID        PRIMARY KEY DEFAULT gen_random_uuid(),
+      titel_de         TEXT,
+      titel_en         TEXT,
+      titel_se         TEXT,
+      status           VARCHAR(20) NOT NULL DEFAULT 'requested'
+                         CHECK (status IN ('requested', 'blocked', 'published')),
+      difficulty_level SMALLINT    CHECK (difficulty_level BETWEEN 1 AND 50),
+      requested_at     TIMESTAMPTZ,
+      published_at     TIMESTAMPTZ,
+      blocked_at       TIMESTAMPTZ,
+      created_at       TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+      updated_at       TIMESTAMPTZ NOT NULL DEFAULT NOW()
     )
   `);
 
+  // Felder nachrüsten falls Tabelle schon als Platzhalter existiert
+  const catCols = ['titel_de TEXT', 'titel_en TEXT', 'titel_se TEXT',
+    "status VARCHAR(20) NOT NULL DEFAULT 'requested'",
+    'difficulty_level SMALLINT', 'requested_at TIMESTAMPTZ',
+    'published_at TIMESTAMPTZ', 'blocked_at TIMESTAMPTZ'];
+  for (const col of catCols) {
+    const name = col.split(' ')[0];
+    await query(`ALTER TABLE categories ADD COLUMN IF NOT EXISTS ${col}`).catch(() => {});
+    // CHECK constraint nur wenn noch nicht vorhanden
+    if (name === 'status') {
+      await query(`ALTER TABLE categories DROP CONSTRAINT IF EXISTS categories_status_check`).catch(() => {});
+      await query(`ALTER TABLE categories ADD CONSTRAINT categories_status_check CHECK (status IN ('requested', 'blocked', 'published'))`).catch(() => {});
+    }
+  }
+
   await query(`
     DROP TRIGGER IF EXISTS categories_updated_at ON categories;
     CREATE TRIGGER categories_updated_at