feat: Tutorial-Fortschritt serverseitig persistieren (Resume/Restart)

- Neuer Router /api/tutorial mit GET/PUT/DELETE für Fortschritt pro User
- DB-Migration: tutorial_step + tutorial_completed in users-Tabelle
- Resume-Dialog bei abgebrochenem Tutorial (Fortsetzen/Neu starten)
- Chat-Hinweis passt sich dem Tutorial-Status dynamisch an
- API-Methoden: getTutorialState, saveTutorialState, resetTutorialState

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/database.py

@@ -464,6 +464,13 @@ async def init_db():
             await db.execute("ALTER TABLE users ADD COLUMN is_active INTEGER DEFAULT 1")
             await db.commit()
 
+        # Migration: Tutorial-Fortschritt pro User
+        if "tutorial_step" not in user_columns:
+            await db.execute("ALTER TABLE users ADD COLUMN tutorial_step INTEGER DEFAULT NULL")
+            await db.execute("ALTER TABLE users ADD COLUMN tutorial_completed INTEGER DEFAULT 0")
+            await db.commit()
+            logger.info("Migration: tutorial_step + tutorial_completed zu users hinzugefuegt")
+
         if "last_login_at" not in user_columns:
             await db.execute("ALTER TABLE users ADD COLUMN last_login_at TIMESTAMP")
             await db.commit()