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

py/services/model_update_service.py

@@ -1362,6 +1362,36 @@ class ModelUpdateService:
             return None
         return float(row["newest"])
 
+    def count_priced_versions(self, model_type: Optional[str] = None) -> int:
+        """Versions with a known price, for the panel's empty state.
+
+        Without it, a user whose threshold is 0 sees "nothing is under your price
+        threshold" while dozens of paid versions already have a price.
+        """
+
+        params: List[Any] = []
+        type_filter = ""
+        if model_type:
+            type_filter = "AND s.model_type = ?"
+            params.append(model_type)
+
+        with self._connect() as conn:
+            row = conn.execute(
+                f"""
+                SELECT COUNT(*) AS priced
+                FROM model_update_versions v
+                JOIN model_update_status s ON s.model_id = v.model_id
+                WHERE v.should_ignore = 0
+                  AND s.should_ignore_model = 0
+                  {type_filter}
+                  AND v.price_buzz IS NOT NULL
+                """,
+                tuple(params),
+            ).fetchone()
+        if row is None:
+            return 0
+        return int(row["priced"])
+
     def count_unavailable_prices(self, model_type: Optional[str] = None) -> int:
         """Gated versions whose price we tried to read and could not.