Drop the pearls currency

Pearls were earned every run and spent on nothing: the aquarium fills up by
finishing worlds, not by paying for it. A counter that only ever goes up is one
more stat competing for attention on the result sheet and the home screen, and
one more field to carry through the run payload, the progress file and both test
suites.

Stars and the animal ladder already say how a run went, so nothing is lost.

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

python-backend/tests/test_tippen_api.py

@@ -56,7 +56,6 @@ def _run_body(lesson_id: str, *, stars: int = 3, passed: bool = True) -> dict:
         "animal": "fish",
         "points": 42.0,
         "passed": passed,
-        "pearls": 5,
         "strokes": [{"key": "a", "expected": "a", "correct": True, "at": 0.0}],
     }
 
@@ -171,7 +170,6 @@ async def test_get_progress_is_fresh_with_only_the_first_lesson_unlocked(
     body = (await client.get("/api/tippen/progress")).json()
     assert body["lessons"]["l01"]["unlocked"] is True
     assert body["lessons"]["l02"]["unlocked"] is False
-    assert body["pearls"] == 0
 
 
 async def test_put_settings_round_trips(client: httpx2.AsyncClient) -> None: