Fix db.py connection handling and test isolation

db.py:
- Add conn.isolation_level = None to get_conn() — fixes row_factory +
  autocommit conflict. row_factory disables implicit transactions,
  so we need explicit autocommit mode.
- Remove all conn.commit() calls (unnecessary with autocommit)

pyproject.toml:
- Move pytest + pytest-asyncio to main dependencies (uv run pytest
  uses ephemeral env with main deps only)

tests/test_db.py:
- Fix test_upsert_user_updates_username to not chain upsert_user()
  calls in assert expressions (test isolation issue)

apps/telegram-bot/tests/test_db.py

@@ -15,9 +15,11 @@ class TestUsers:
         assert row["username"] == "alice"
 
     def test_upsert_user_updates_username(self):
-        _db.upsert_user(123, "alice")
+        # Two upserts to the same telegram_user_id: second one updates the username.
+        # Returns the same id both times (idempotent).
+        uid1 = _db.upsert_user(123, "alice")
         uid2 = _db.upsert_user(123, "alice_updated")
-        assert uid2 == _db.upsert_user(123, "alice")  # same id
+        assert uid1 == uid2
         row = _db.get_user_by_telegram_id(123)
         assert row["username"] == "alice_updated"