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/css/components/lora-modal/versions.css

@@ -460,6 +460,20 @@
     border-color: color-mix(in oklch, #845EF7 45%, transparent);
 }
 
+/* Price alert badge - green tone (#37B24D) for a price under the user's threshold */
+.version-badge-price-alert {
+    background: color-mix(in oklch, #37B24D 25%, transparent);
+    color: #2B8A3E;
+    border-color: color-mix(in oklch, #37B24D 55%, transparent);
+    font-variant-numeric: tabular-nums;
+}
+
+[data-theme="dark"] .version-badge-price-alert {
+    background: color-mix(in oklch, #37B24D 20%, transparent);
+    color: #69DB7C;
+    border-color: color-mix(in oklch, #37B24D 45%, transparent);
+}
+
 .version-meta-ea {
     color: #E67700;
     font-weight: 600;