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/routes/handlers/misc_handlers.py

@@ -4385,6 +4385,7 @@ class PriceAlertsHandler:
             )
             newest_checked_at = update_service.newest_price_checked_at()
             unavailable_count = update_service.count_unavailable_prices()
+            priced_count = update_service.count_priced_versions()
         except Exception as exc:
             logger.error("Failed to load price alerts: %s", exc, exc_info=True)
             return web.json_response(
@@ -4410,6 +4411,9 @@ class PriceAlertsHandler:
                 # Gated versions whose price could not be read (mature models
                 # are served only by the challenged civitai.red host).
                 "unavailableCount": unavailable_count,
+                # Lets the empty state explain a 0 Buzz threshold instead of
+                # claiming there is simply nothing to show.
+                "pricedCount": priced_count,
                 "alerts": alerts,
             }
         )