fix(ui): explain a 0 Buzz threshold in the price alerts panel

The default threshold is 0 ("only tell me when a version becomes free") and the
settings copy says so, but the panel did not: a real instance with 52 priced paid
versions and an untouched threshold showed "Nothing is under your price threshold
right now" with only a small "Alert threshold: 0 Buzz" in the corner, which reads
as a broken feature.

- the payload now carries pricedCount, so the empty state can say how many paid
  versions already have a known price
- when the threshold is 0 and prices are known, the empty state says so and
  points at Settings - Library instead of implying there is nothing to show
- the read-time threshold comparison means setting one takes effect immediately;
  measured on a copy of that instance: 0 Buzz -> 0 alerts, 100 -> 44,
  500 -> 48, 5000 -> 51

tests/routes/test_misc_routes.py

@@ -2950,10 +2950,11 @@ def _price_alerts_adapter(update_service, scanners=None):
 
 
 class _FakeUpdateService:
-    def __init__(self, alerts, *, unavailable_count=0):
+    def __init__(self, alerts, *, unavailable_count=0, priced_count=0):
         self.alerts = alerts
         self.calls = []
         self.unavailable_count = unavailable_count
+        self.priced_count = priced_count
 
     async def get_price_alerts(self, model_type=None, *, threshold_buzz=None, limit=200):
         self.calls.append((model_type, threshold_buzz, limit))
@@ -2965,6 +2966,9 @@ class _FakeUpdateService:
     def count_unavailable_prices(self, model_type=None):
         return self.unavailable_count
 
+    def count_priced_versions(self, model_type=None):
+        return self.priced_count
+
 
 @pytest.mark.asyncio
 async def test_price_alerts_handler_returns_the_global_list():
@@ -3002,6 +3006,7 @@ async def test_price_alerts_handler_returns_the_global_list():
     assert payload["thresholdBuzz"] == 300
     assert payload["newestCheckedAt"] == 1791039694.5
     assert payload["unavailableCount"] == 0
+    assert payload["pricedCount"] == 0
     assert payload["alerts"][0]["civitaiUrl"] == (
         "https://civitai.com/models/2981320?modelVersionId=3379626"
     )