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/en.json

@@ -201,7 +201,12 @@
       "loading": "Checking for {type} updates...",
       "success": "Found {count} update(s) for {type}s",
       "none": "All {type}s are up to date",
-      "error": "Failed to check for {type} updates: {message}"
+      "error": "Failed to check for {type} updates: {message}",
+      "gateEvents": {
+        "becameFree": "{count} version(s) became free",
+        "priceDrop": "{count} version(s) dropped below your price threshold",
+        "newGate": "{count} version(s) are now paid"
+      }
     },
     "cleanupExampleImages": {
       "label": "Clean up example image folders",
@@ -387,6 +392,7 @@
       "downloadPathTemplates": "Download Path Templates",
       "priorityTags": "Priority Tags",
       "versionScope": "Version Scope",
+      "priceTracking": "Buzz Price Tracking",
       "exampleImages": "Example Images",
       "autoOrganize": "Auto-organize",
       "metadata": "Metadata",
@@ -753,6 +759,16 @@
       "label": "Hide Paid Updates",
       "help": "When enabled, models with only paid updates will not show 'Update available' badge"
     },
+    "priceTracking": {
+      "label": "Buzz Price Tracking",
+      "help": "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": "Track Buzz prices for paid versions",
+      "enabledHelp": "Show the price on paid and early access versions, and alert when a price drops below your threshold",
+      "threshold": "Price alert threshold (Buzz)",
+      "thresholdHelp": "Alert when a paid version costs this much or less. Use 0 to only be told when a version becomes free.",
+      "refreshHours": "Price refresh interval (hours)",
+      "refreshHoursHelp": "How long a stored price stays fresh before it is fetched again"
+    },
     "licenseIcons": {
       "useNewStyle": "Use updated license icons",
       "useNewStyleHelp": "Display license permissions with colored indicators (new style) or restriction-only icons (classic style). Mirroring the current CivitAI design."
@@ -2008,6 +2024,11 @@
           "earlyAccessTooltip": "This version currently requires CivitAI early access",
           "paid": "Paid",
           "paidTooltip": "This version requires payment to download",
+          "priceSaleTooltip": "On sale, normally {list} Buzz",
+          "priceBlueBuzzTooltip": "Can also be paid with Blue Buzz",
+          "priceAlertTooltip": "{price} - below your price alert threshold",
+          "freeNow": "Free Now",
+          "freeNowTooltip": "This version no longer requires payment",
           "ignored": "Ignored",
           "ignoredTooltip": "Update notifications are disabled for this version",
           "onSiteOnly": "On-Site Only",