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

locales/de.json

@@ -234,6 +234,9 @@
         },
         "groupByModel": {
             "label": "Nach Modell gruppieren"
+        },
+        "priceAlerts": {
+            "label": "[TODO: Translate] Price alerts"
         }
     },
     "header": {
@@ -942,7 +945,9 @@
                 "action": "Updates",
                 "menuLabel": "Weitere Update-Optionen anzeigen",
                 "check": "Updates prüfen",
-                "checkTooltip": "Die Aktualisierungssuche kann einige Zeit dauern."
+                "checkTooltip": "Die Aktualisierungssuche kann einige Zeit dauern.",
+                "priceAlerts": "[TODO: Translate] Price alerts",
+                "priceAlertsTooltip": "[TODO: Translate] Buzz price alerts for paid versions"
             }
         },
         "bulkOperations": {
@@ -2321,7 +2326,8 @@
         "notificationsTitle": "Benachrichtigungszentrum",
         "tabs": {
             "updates": "Aktualisierungen",
-            "messages": "Mitteilungen"
+            "messages": "Mitteilungen",
+            "priceAlerts": "[TODO: Translate] Price Alerts"
         },
         "updateAvailable": "Update verfügbar",
         "noChangelogAvailable": "Kein detailliertes Changelog verfügbar. Weitere Informationen auf GitHub.",
@@ -2377,6 +2383,32 @@
             "shown": "{time} angezeigt",
             "dismissed": "{time} geschlossen",
             "active": "Aktiv"
+        },
+        "priceAlerts": {
+            "disabled": "[TODO: Translate] CivitAI does not publish prices in its public API, so prices are read from the model page. Turn on price tracking to see them here.",
+            "openSettings": "[TODO: Translate] Open settings",
+            "segmentsLabel": "[TODO: Translate] Alert kind",
+            "segments": {
+                "belowThreshold": "[TODO: Translate] Under threshold",
+                "becameFree": "[TODO: Translate] Became free"
+            },
+            "thresholdLabel": "[TODO: Translate] Alert threshold: {buzz} Buzz",
+            "empty": "[TODO: Translate] Nothing is under your price threshold right now.",
+            "emptyFree": "[TODO: Translate] No version has become free yet.",
+            "loadFailed": "[TODO: Translate] Could not refresh the alerts; showing the last known list.",
+            "lastChecked": "[TODO: Translate] Prices last checked {when}",
+            "justNow": "[TODO: Translate] just now",
+            "minutesAgo": "[TODO: Translate] {count} min ago",
+            "hoursAgo": "[TODO: Translate] {count} h ago",
+            "daysAgo": "[TODO: Translate] {count} d ago",
+            "freeNow": "[TODO: Translate] Free now",
+            "blueBuzz": "[TODO: Translate] Blue Buzz OK",
+            "earlyAccessUntil": "[TODO: Translate] Early access until {date}",
+            "inLibrary": "[TODO: Translate] In library",
+            "notInLibrary": "[TODO: Translate] Not in library",
+            "droppedAgo": "[TODO: Translate] dropped {when}",
+            "openCivitai": "[TODO: Translate] Open on CivitAI",
+            "openLocal": "[TODO: Translate] Open"
         }
     },
     "support": {