DC-101: Disk Space Monitor + Product Vision

Backend:
- src/monitoring/disk-space-monitor.js: monitors Docker disk usage against
  user-configured budget, auto-cleans at thresholds, breaks down by category
- routes/disk-space.js: GET /disk, GET /disk/breakdown, POST /disk/config,
  POST /disk/cleanup endpoints
- src/app.js: wire DiskSpaceMonitor into startup, 10-min check interval
- All 1539 tests pass

Product Vision (PRODUCT-VISION.md):
- DashCaddy is a self-hosting platform, not just a dashboard
- Core value: 'Self-host anything in 30 seconds'
- Three pillars: One-click deploy, zero-config networking, self-healing infra
- vs Portainer/CasaOS/Yunohost positioning

New backlog tasks (P5 tier, DC-101–108):
- Disk budget, one-click deploy with auto Caddyfile+DNS, container
  auto-discovery, app catalog, smart wizard, visual Caddy builder,
  disaster recovery, multi-host fleet management

47 total backlog tasks, ~110 hr of work, cron running every 2h.

dashcaddy-api/src/app.js

@@ -90,9 +90,11 @@ const DependencyManager = require('./managers/dependency-manager');
 const autoRestartRoutes = require('../routes/auto-restart');
 const configDriftRoutes = require('../routes/config-drift');
 const sslMonitorRoutes = require('../routes/ssl-monitor');
+const diskSpaceRoutes = require('../routes/disk-space');
 const { AutoRestartManager } = require('./managers/auto-restart-manager');
 const { ConfigDriftDetector } = require('./managers/config-drift-detector');
 const SSLMonitor = require('./monitoring/ssl-monitor');
+const { DiskSpaceMonitor } = require('./monitoring/disk-space-monitor');
 const DNSPropagationChecker = require('./dns/dns-propagation');
 
 // Constants
@@ -455,6 +457,12 @@ async function createApp() {
   sslMonitor.start(3600000); // 1 hour
   log.info('app', 'SSL monitor initialized');
 
+  // Initialize disk space monitor (disk budget + auto-cleanup)
+  const diskSpaceMonitor = new DiskSpaceMonitor({ log, config: ctx.siteConfig });
+  ctx.diskSpaceMonitor = diskSpaceMonitor;
+  diskSpaceMonitor.start(600000); // 10 min
+  log.info('app', 'Disk space monitor initialized', { budgetGB: diskSpaceMonitor.getConfig().diskBudgetGB });
+
   // Initialize DNS propagation checker
   const dnsPropagationChecker = new DNSPropagationChecker(ctx);
   ctx.dnsPropagationChecker = dnsPropagationChecker;
@@ -709,6 +717,11 @@ async function createApp() {
     asyncHandler: ctx.asyncHandler,
     logError: ctx.logError,
   }));
+  apiRouter.use('/disk', diskSpaceRoutes({
+    diskSpaceMonitor: ctx.diskSpaceMonitor,
+    asyncHandler: ctx.asyncHandler,
+    log: ctx.log,
+  }));
 
   // Inline API routes (mounted under /api/v1 below)
   // Note: /health lives at root only — see root-level health check below.