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/components/shared/ModelVersionsTab.js

@@ -187,6 +187,53 @@ function isPaidPermanent(version) {
     return version && version.isPaid === true;
 }
 
+function isGated(version) {
+    return isEarlyAccessActive(version) || isPaidPermanent(version);
+}
+
+/**
+ * Format a Buzz price for display, or '' when the price is unknown (price
+ * tracking is opt-in, so most versions have no price at all).
+ */
+function formatBuzzPrice(value) {
+    if (typeof value !== 'number' || !Number.isFinite(value) || value < 0) {
+        return '';
+    }
+    return `${value.toLocaleString()} Buzz`;
+}
+
+/**
+ * Tooltip for a known price: notes a sale against the list price and the Blue
+ * Buzz option, since both change what the number means to a buyer.
+ */
+function buildPriceTooltip(version, baseTooltip) {
+    const effective = formatBuzzPrice(version?.priceBuzz);
+    const segments = [baseTooltip];
+    if (!effective) {
+        return baseTooltip;
+    }
+    const list = version?.listPriceBuzz;
+    if (typeof list === 'number' && Number.isFinite(list) && list > version.priceBuzz) {
+        segments.push(
+            translate(
+                'modals.model.versions.badges.priceSaleTooltip',
+                { list: list.toLocaleString() },
+                `On sale, normally ${list.toLocaleString()} Buzz`
+            )
+        );
+    }
+    if (version?.acceptsBlueBuzz) {
+        segments.push(
+            translate(
+                'modals.model.versions.badges.priceBlueBuzzTooltip',
+                {},
+                'Can also be paid with Blue Buzz'
+            )
+        );
+    }
+    return segments.join(' · ');
+}
+
 function isDownloadAllowed(version) {
     if (!version.usageControl) {
         return true;
@@ -524,20 +571,55 @@ function renderRow(version, options) {
 
     if (isEarlyAccess) {
         badges.push(buildBadge(earlyAccessBadgeLabel, 'early-access', {
-            title: translate(
-                'modals.model.versions.badges.earlyAccessTooltip',
-                {},
-                'This version currently requires Civitai early access'
+            title: buildPriceTooltip(
+                version,
+                translate(
+                    'modals.model.versions.badges.earlyAccessTooltip',
+                    {},
+                    'This version currently requires Civitai early access'
+                )
             ),
         }));
     }
 
     if (isPaidPermanent(version)) {
         badges.push(buildBadge(paidBadgeLabel, 'paid', {
+            title: buildPriceTooltip(
+                version,
+                translate(
+                    'modals.model.versions.badges.paidTooltip',
+                    {},
+                    'This version requires payment to download'
+                )
+            ),
+        }));
+    }
+
+    // Known price for a gated version. The price is only captured when the
+    // optional price tracking is enabled, so this stays hidden otherwise.
+    const priceLabel = isGated(version) ? formatBuzzPrice(version.priceBuzz) : '';
+    if (priceLabel) {
+        const alertsEnabled = !!version.priceAlert;
+        badges.push(buildBadge(priceLabel, alertsEnabled ? 'price-alert' : 'paid', {
+            title: alertsEnabled
+                ? translate(
+                    'modals.model.versions.badges.priceAlertTooltip',
+                    { price: priceLabel },
+                    `${priceLabel} - below your price alert threshold`
+                )
+                : buildPriceTooltip(version, paidBadgeLabel),
+        }));
+    }
+
+    // A version that used to be gated and no longer is. `gateLapsedAt` is
+    // persisted, so the marker survives long after the refresh that saw it.
+    if (!isGated(version) && version.gateLapsedAt) {
+        const freeBadgeLabel = translate('modals.model.versions.badges.freeNow', {}, 'Free Now');
+        badges.push(buildBadge(freeBadgeLabel, 'success', {
             title: translate(
-                'modals.model.versions.badges.paidTooltip',
-                {},
-                'This version requires payment to download'
+                'modals.model.versions.badges.freeNowTooltip',
+                { date: formatDateLabel(version.gateLapsedAt) || '' },
+                'This version no longer requires payment'
             ),
         }));
     }