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.

locales/fr.json

@@ -201,7 +201,12 @@
             "loading": "Recherche de mises à jour pour {type}...",
             "success": "{count} mise(s) à jour trouvée(s) pour {type}",
             "none": "Tous les {type} sont à jour",
-            "error": "Échec de la vérification des mises à jour pour {type} : {message}"
+            "error": "Échec de la vérification des mises à jour pour {type} : {message}",
+            "gateEvents": {
+                "becameFree": "[TODO: Translate] {count} version(s) became free",
+                "priceDrop": "[TODO: Translate] {count} version(s) dropped below your price threshold",
+                "newGate": "[TODO: Translate] {count} version(s) are now paid"
+            }
         },
         "cleanupExampleImages": {
             "label": "Supprimer les dossiers d'exemples orphelins",
@@ -387,6 +392,7 @@
             "downloadPathTemplates": "Modèles de chemin de téléchargement",
             "priorityTags": "Tags prioritaires",
             "versionScope": "Indicateurs de mise à jour",
+            "priceTracking": "[TODO: Translate] Buzz Price Tracking",
             "exampleImages": "Images d'exemple",
             "autoOrganize": "Organisation automatique",
             "metadata": "Métadonnées",
@@ -753,6 +759,16 @@
             "label": "Masquer les mises à jour payantes",
             "help": "Lorsque cette option est activée, les modèles n'ayant que des mises à jour payantes n'affichent pas le badge « Mise à jour disponible »"
         },
+        "priceTracking": {
+            "label": "[TODO: Translate] Buzz Price Tracking",
+            "help": "[TODO: Translate] CivitAI does not publish prices in its public API, so prices are read from the model page - one extra request per paid model, refreshed on the interval below. Requires an update check to pick up changes.",
+            "enabled": "[TODO: Translate] Track Buzz prices for paid versions",
+            "enabledHelp": "[TODO: Translate] Show the price on paid and early access versions, and alert when a price drops below your threshold",
+            "threshold": "[TODO: Translate] Price alert threshold (Buzz)",
+            "thresholdHelp": "[TODO: Translate] Alert when a paid version costs this much or less. Use 0 to only be told when a version becomes free.",
+            "refreshHours": "[TODO: Translate] Price refresh interval (hours)",
+            "refreshHoursHelp": "[TODO: Translate] How long a stored price stays fresh before it is fetched again"
+        },
         "licenseIcons": {
             "useNewStyle": "Utiliser les icônes de licence mises à jour",
             "useNewStyleHelp": "Afficher les permissions de licence avec des indicateurs colorés (nouveau style) ou des icônes de restriction uniquement (style classique). Reprend le design actuel de CivitAI."
@@ -2008,6 +2024,11 @@
                     "earlyAccessTooltip": "Cette version nécessite actuellement l'accès anticipé CivitAI",
                     "paid": "Payant",
                     "paidTooltip": "Cette version nécessite un paiement pour être téléchargée",
+                    "priceSaleTooltip": "[TODO: Translate] On sale, normally {list} Buzz",
+                    "priceBlueBuzzTooltip": "[TODO: Translate] Can also be paid with Blue Buzz",
+                    "priceAlertTooltip": "[TODO: Translate] {price} - below your price alert threshold",
+                    "freeNow": "[TODO: Translate] Free Now",
+                    "freeNowTooltip": "[TODO: Translate] This version no longer requires payment",
                     "ignored": "Ignorée",
                     "ignoredTooltip": "Les notifications de mise à jour sont désactivées pour cette version",
                     "onSiteOnly": "Uniquement sur Site",