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.

py/services/settings_manager.py

@@ -129,6 +129,11 @@ DEFAULT_SETTINGS: Dict[str, Any] = {
     "model_card_footer_action": "replace_preview",
     "show_version_on_card": True,
     "version_grouping": "same_base",
+    # Buzz price tracking for paid/early-access versions. Opt-in because reading a
+    # price costs one extra (public) model-page request per gated model.
+    "price_tracking_enabled": False,
+    "price_alert_threshold_buzz": 0,  # 0 = alert on "became free" only
+    "price_check_ttl_hours": 24,
     "auto_organize_exclusions": [],
     "metadata_refresh_skip_paths": [],
     "skip_previously_downloaded_model_versions": False,