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

templates/components/modals/update_modal.html

@@ -15,6 +15,10 @@
                 <span>{{ t('update.tabs.messages') }}</span>
                 <span class="notification-tab-badge" id="bannerTabBadge"></span>
             </button>
+            <button type="button" class="notification-tab" id="priceAlertsTab" role="tab" aria-selected="false" aria-controls="priceAlertsPanel" tabindex="-1" data-notification-tab="priceAlerts">
+                <span>{{ t('update.tabs.priceAlerts') }}</span>
+                <span class="notification-tab-badge" id="priceAlertsTabBadge"></span>
+            </button>
         </div>
         <div class="notification-panels">
             <div class="notification-panel active" id="updatesPanel" role="tabpanel" aria-labelledby="updatesTab" aria-hidden="false" tabindex="0" data-notification-panel="updates">
@@ -94,6 +98,34 @@
                     <ul class="banner-history-list" id="bannerHistoryList"></ul>
                 </div>
             </div>
+            <div class="notification-panel" id="priceAlertsPanel" role="tabpanel" aria-labelledby="priceAlertsTab" aria-hidden="true" tabindex="-1" data-notification-panel="priceAlerts">
+                <div class="price-alerts">
+                    <!-- Shown while price tracking is off: the panel still lists
+                         versions that became free, but says why prices are missing. -->
+                    <div class="price-alerts-disabled hidden" id="priceAlertsDisabled">
+                        <p class="price-alerts-disabled-text">{{ t('update.priceAlerts.disabled') }}</p>
+                        <button type="button" class="price-alerts-settings-btn" id="priceAlertsSettingsBtn">
+                            <i class="fas fa-sliders-h"></i> {{ t('update.priceAlerts.openSettings') }}
+                        </button>
+                    </div>
+
+                    <div class="price-alerts-toolbar">
+                        <div class="price-alerts-segments" role="tablist" aria-label="{{ t('update.priceAlerts.segmentsLabel') }}">
+                            <button type="button" class="price-alerts-segment active" data-price-alert-segment="below_threshold" role="tab" aria-selected="true">
+                                {{ t('update.priceAlerts.segments.belowThreshold') }}
+                            </button>
+                            <button type="button" class="price-alerts-segment" data-price-alert-segment="became_free" role="tab" aria-selected="false">
+                                {{ t('update.priceAlerts.segments.becameFree') }}
+                            </button>
+                        </div>
+                        <span class="price-alerts-threshold" id="priceAlertsThreshold"></span>
+                    </div>
+
+                    <p class="price-alerts-note hidden" id="priceAlertsStale"></p>
+                    <p class="price-alerts-empty" id="priceAlertsEmpty">{{ t('update.priceAlerts.empty') }}</p>
+                    <ul class="price-alerts-list" id="priceAlertsList"></ul>
+                </div>
+            </div>
         </div>
     </div>
 </div>