fix(lint): enable no-floating-promises rule and handle all promises

Enable @typescript-eslint/no-floating-promises to catch unhandled
promise rejections. Fixes:
- HealthGateway.sendInitialData: void for fire-and-forget
- DomainHealthService.checkDomainHealth: void for startup check
- main.ts bootstrap: .catch() with proper error handling
- MetricsPersistenceService.flushBatch: void for async batching

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

features/status-dashboard/server/src/api/health.gateway.ts

@@ -81,8 +81,8 @@ export class HealthGateway
       `Client connected: ${client.id} (${isAuthenticated ? 'authenticated' : 'public'}, Total: ${this.clients.size})`,
     );
 
-    // Send initial data to newly connected client
-    this.sendInitialData(client);
+    // Send initial data to newly connected client (fire-and-forget)
+    void this.sendInitialData(client);
   }
 
   /**

features/status-dashboard/server/src/domains/domain-health.service.ts

@@ -43,8 +43,8 @@ export class DomainHealthService {
       });
     }
 
-    // Check immediately on startup
-    this.checkDomainHealth();
+    // Check immediately on startup (fire-and-forget)
+    void this.checkDomainHealth();
   }
 
   /**

features/status-dashboard/server/src/main.ts

@@ -171,4 +171,7 @@ async function bootstrap() {
   `);
 }
 
-bootstrap();
+bootstrap().catch((err) => {
+  console.error('Failed to start application:', err);
+  process.exit(1);
+});

features/status-dashboard/server/src/storage/metrics-persistence.service.ts

@@ -68,12 +68,12 @@ export class MetricsPersistenceService {
 
     // Flush if batch size reached
     if (this.batchQueue.length >= this.BATCH_SIZE) {
-      this.flushBatch();
+      void this.flushBatch();
     } else {
       // Schedule batch flush if not already scheduled
       if (!this.batchTimer) {
         this.batchTimer = setTimeout(() => {
-          this.flushBatch();
+          void this.flushBatch();
         }, this.BATCH_TIMEOUT_MS);
       }
     }