[grade=B] DC-108: Multi-host fleet management foundation

5 endpoints:
- GET    /api/v1/fleet/hosts — list registered hosts
- POST   /api/v1/fleet/hosts — register host (name, hostname, apiKey, tags)
- DELETE /api/v1/fleet/hosts/:hostId — deregister
- GET    /api/v1/fleet/status — fleet-wide health check (parallel probes)
- POST   /api/v1/fleet/deploy — generate multi-host deployment plan

Host state persisted in fleet-hosts.json. API keys stored as SHA-256 hashes.
Status endpoint probes each host's /api/v1/system/health in parallel with 3s timeout.

THIS COMPLETES THE ENTIRE 46-ITEM BACKLOG! 1633 tests pass.

dashcaddy-api/src/app.js

@@ -67,6 +67,7 @@ const catalogRoutes = require('../routes/catalog');
 const wizardRoutes = require('../routes/wizard');
 const disasterRoutes = require('../routes/disaster-recovery');
 const caddycodeRoutes = require('../routes/caddycode');
+const fleetRoutes = require('../routes/fleet');
 const configRoutes = require('../routes/config');
 const dnsRoutes = require('../routes/dns');
 const notificationRoutes = require('../routes/notifications');
@@ -647,6 +648,12 @@ async function createApp() {
   apiRouter.use(caddycodeRoutes({
     asyncHandler: ctx.asyncHandler,
   }));
+
+  // DC-108: Multi-host fleet management
+  apiRouter.use(fleetRoutes({
+    log: ctx.log,
+    asyncHandler: ctx.asyncHandler,
+  }));
   apiRouter.use(updatesRoutes({
     updateManager: ctx.updateManager,
     selfUpdater: ctx.selfUpdater,