feat: Fortschritts-Tracking – user_daily_activity, Tagesziel & GET /auth/stats

- Neue Tabelle user_daily_activity (Tagesverlauf) + Spalte daily_goal_ep
- POST /auth/progress schreibt Tagesaktivität mit
- GET /auth/me liefert daily_goal_ep
- Neuer GET /auth/stats: Tagesverlauf, Tagesziel, Totals, echte Skills je answer_type
- Neuer PUT /auth/goal zum Setzen des Tagesziels

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/db-migrate.js

@@ -534,6 +534,21 @@ async function migrate() {
       FOR EACH ROW EXECUTE FUNCTION update_last_seen_at()
   `);
 
+  // user_daily_activity — Tagesverlauf für Streak-Kalender, Wochengraph, Tagesziel
+  await query(`
+    CREATE TABLE IF NOT EXISTS user_daily_activity (
+      user_id       UUID    NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+      activity_date DATE    NOT NULL,
+      ep_earned     INTEGER NOT NULL DEFAULT 0,
+      cards_done    INTEGER NOT NULL DEFAULT 0,
+      correct_count INTEGER NOT NULL DEFAULT 0,
+      PRIMARY KEY (user_id, activity_date)
+    )
+  `);
+
+  // Tagesziel (EP/Tag) auf dem App-Profil
+  await query(`ALTER TABLE users_public ADD COLUMN IF NOT EXISTS daily_goal_ep INTEGER NOT NULL DEFAULT 30`).catch(() => {});
+
   // audios
   await query(`
     CREATE TABLE IF NOT EXISTS audios (