feat(update): add a Buzz price alerts panel to the notification bell

P5a of docs/plans/paid-model-price-tracking.md: one surface that answers "what
got cheaper / became free", without a permanent button (the grid filter was
dropped by owner decision, so the panel carries the actions itself).

- price_alert_since records when an alert started, so the panel can say
  "dropped 3 d ago" and count what is new since the user last looked; it is set
  on the first sight of an already-cheap version, preserved while the alert
  stands, and cleared when the price rises back above the threshold
- get_price_alerts() compares the threshold at read time (editing it takes
  effect immediately, no refresh needed) and returns both kinds in one list;
  model_type=None covers every type, which the shared update DB makes a single
  query. "became free" needs no price data, so it is reported even while price
  tracking is off
- GET /api/lm/price-alerts, registered once in MiscRoutes rather than per model
  type, decorating rows best-effort with the local model name and file path from
  the scanner indexes (a cold cache just omits them)
- a third tab in the notification bell: segments for under-threshold and
  became-free, the three states (tracking off / nothing matching / stale), and
  per-row actions (CivitAI always, Open when the model is local)
- two non-permanent entry points share one helper: the controls-bar updates
  dropdown and the global context menu, whose label carries the unread count
- unread state stays client-side (localStorage watermark); the count is fetched
  once on init and only when price tracking is enabled
- the per-type frontend client method is removed as dead code; the per-type
  backend route stays for the companion extension

py/routes/misc_routes.py

@@ -31,6 +31,7 @@ from .handlers.misc_handlers import (
     ModelLibraryHandler,
     NodeRegistry,
     NodeRegistryHandler,
+    PriceAlertsHandler,
     SettingsHandler,
     SidecarMigrationHandler,
     SupportersHandler,
@@ -140,6 +141,10 @@ class MiscRoutes:
         doctor = DoctorHandler(settings_service=self._settings)
         example_workflows = ExampleWorkflowsHandler()
         base_model = BaseModelHandlerSet()
+        price_alerts = PriceAlertsHandler(
+            settings_service=self._settings,
+            service_registry=self._service_registry_adapter,
+        )
         model_source_handler = ModelSourceHandler()
         agent_handler = AgentHandler()
         download_routing = DownloadRoutingHandler()
@@ -163,6 +168,7 @@ class MiscRoutes:
             doctor=doctor,
             example_workflows=example_workflows,
             base_model=base_model,
+            price_alerts=price_alerts,
             model_source_handler=model_source_handler,
             agent_handler=agent_handler,
             download_routing=download_routing,