feat(update): track buzz prices and alert below a threshold

CivitAI's public API deliberately omits prices — paidAccess is trimmed to
{permanent, endsAt} because "pricing belongs to the purchase flow" — but the
public model page embeds the site's own model.getById result, including
paidAccess.terms, in its server-rendered payload. That is read anonymously
(no API key, no internal endpoint, no forged Origin), one request per gated
model, so only the ~2% of models that actually carry a gate pay for it.

- optional capture, off by default: price_tracking_enabled,
  price_alert_threshold_buzz (0 = alert on "became free" only) and
  price_check_ttl_hours; prices refresh on their own TTL and immediately when a
  gate changes, and a failed fetch keeps the stored price instead of blanking it
- versions that stop carrying a gate are marked free (persisted gate_lapsed_at)
  and gate transitions are reported as events on the refresh response, so a
  version already in the library can announce that it became free
- price_alert_state plus a price_drop edge event; new
  GET /api/lm/{type}/updates/price-alerts lists what is under the threshold
- versions tab shows the price (effective, with the list price struck through
  and a Blue Buzz note) and a Free Now badge; an update check toasts the
  transitions in one message
- the parser and the alerts query are unit-tested against a trimmed page
  fixture, and every route definition is now asserted to resolve to a handler

Plan, verification notes and the deviations from it are in
docs/plans/paid-model-price-tracking.md.

static/js/managers/SettingsManager.js

@@ -1163,6 +1163,22 @@ export class SettingsManager {
             hidePaidUpdatesCheckbox.checked = state.global.settings.hide_paid_updates || false;
         }
 
+        // Set buzz price tracking settings
+        const priceTrackingEnabledCheckbox = document.getElementById('priceTrackingEnabled');
+        if (priceTrackingEnabledCheckbox) {
+            priceTrackingEnabledCheckbox.checked = state.global.settings.price_tracking_enabled || false;
+        }
+
+        const priceAlertThresholdInput = document.getElementById('priceAlertThresholdBuzz');
+        if (priceAlertThresholdInput) {
+            priceAlertThresholdInput.value = state.global.settings.price_alert_threshold_buzz ?? 0;
+        }
+
+        const priceCheckTtlInput = document.getElementById('priceCheckTtlHours');
+        if (priceCheckTtlInput) {
+            priceCheckTtlInput.value = state.global.settings.price_check_ttl_hours ?? 24;
+        }
+
         const skipPreviouslyDownloadedModelVersionsCheckbox = document.getElementById('skipPreviouslyDownloadedModelVersions');
         if (skipPreviouslyDownloadedModelVersionsCheckbox) {
             skipPreviouslyDownloadedModelVersionsCheckbox.checked =