feat(notifications): add structured category enum to dispatcher and history (#774)

Introduce a NotificationCategory string-literal union (11 values) and
thread it through dispatchAlert as a required second argument. All
callers (DockerEventService, AutoHealService, ImageUpdateService,
MonitorService, PolicyEnforcement, policyGate, SchedulerService,
imageUpdates route) pass an explicit category at every call site,
giving TypeScript compile-time enforcement that no new emit site can
be added without choosing a category.

DatabaseService gains an idempotent migration that adds a nullable
category TEXT column to notification_history; existing rows keep
category=NULL (displayed as Uncategorized in the UI). The
getNotificationHistory method accepts an optional category filter
that is forwarded from the GET /api/notifications/history route via
a ?category= query param.

NotificationPanel gains a category Select dropdown so users can
filter history by category. The frontend types mirror the backend
union so API responses are type-safe end-to-end.

All 75 test files (1410 tests) updated to the new 4-arg dispatchAlert
signature and passing.

backend/src/helpers/policyGate.ts

@@ -81,8 +81,9 @@ export async function triggerPostDeployScan(
         if (scan.critical_count > 0 || scan.high_count > 0) {
           NotificationService.getInstance().dispatchAlert(
             scan.critical_count > 0 ? 'error' : 'warning',
+            'scan_finding',
             `Vulnerability scan for ${imageRef}: ${scan.critical_count} critical, ${scan.high_count} high`,
-            stackName,
+            { stackName },
           );
         }
       } catch (err) {
@@ -90,8 +91,9 @@ export async function triggerPostDeployScan(
         console.error(`[Security] Post-deploy scan failed for ${imageRef}:`, message);
         NotificationService.getInstance().dispatchAlert(
           'warning',
+          'scan_finding',
           `Post-deploy scan failed for ${imageRef} (${stackName}): ${message}`,
-          stackName,
+          { stackName },
         );
       }
     }