fix(session): price the balance from the warm rate cache only

/session gates the terminal unlocking, and satoshis_amount_as_fiat() on a
cold cache queries external exchanges (~1 s measured on l484). Read the
LNbits btc-price cache directly instead: warm → fiat, miss → null and
the terminal prices the sats itself. No rate lookup ever blocks a tap.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/test_session.py

@@ -106,16 +106,16 @@ def stubs(monkeypatch):
     async def get_wallet(wallet_id):
         return state["wallet"]
 
-    async def satoshis_amount_as_fiat(amount, currency):
-        assert currency == "USD"
-        return amount * 0.0008  # 123456 sats → 98.7648
+    def cache_get(key, default=None):
+        # A warm LNbits rate cache: 1 BTC = 80,000 USD → 123456 sats = 98.7648
+        return 80_000.0 if key == "btc-price-USD" else default
 
     monkeypatch.setattr(v, "get_card_by_external_id", get_card_by_external_id)
     monkeypatch.setattr(v, "update_card_counter", update_card_counter)
     monkeypatch.setattr(v, "get_hits_today", get_hits_today)
     monkeypatch.setattr(v, "create_hit", create_hit)
     monkeypatch.setattr(v, "get_wallet", get_wallet)
-    monkeypatch.setattr(v, "satoshis_amount_as_fiat", satoshis_amount_as_fiat)
+    monkeypatch.setattr(v.cache, "get", cache_get)
     monkeypatch.setattr(
         v.settings, "lnbits_default_accounting_currency", state["default_currency"]
     )
@@ -209,18 +209,18 @@ async def test_session_falls_back_to_instance_default_currency(stubs, monkeypatc
     p, c = make_sun(counter=6)
     out = await v.api_session(p, c, FakeRequest(), "ext123")
     assert out["currency"] == "USD"
-    assert out["fiat"] == pytest.approx(5 * 0.0008)
+    assert out["fiat"] == pytest.approx(5 / 100_000_000 * 80_000)
 
 
 @pytest.mark.asyncio
-async def test_session_survives_rate_failure(stubs, monkeypatch):
-    async def boom(amount, currency):
-        raise ValueError("no rate")
-
-    monkeypatch.setattr(v, "satoshis_amount_as_fiat", boom)
+async def test_session_leaves_fiat_null_on_a_cold_rate_cache(stubs, monkeypatch):
+    # No external rate lookup ever happens on the unlock path: a cache miss
+    # simply means the terminal prices the balance itself.
+    monkeypatch.setattr(v.cache, "get", lambda key, default=None: default)
     p, c = make_sun(counter=6)
     out = await v.api_session(p, c, FakeRequest(), "ext123")
     assert out["authenticated"] is True
+    assert out["currency"] == "USD"
     assert out["fiat"] is None