[grade=B] DC-097+DC-092: Prometheus metrics export + dependency health checks

DC-097: Add /api/v1/metrics/prometheus endpoint returning standard
Prometheus text exposition format. Includes uptime, request counts
by status/method, error counts, business metrics, memory gauges.
Public (no auth) for Prometheus scraping.

DC-092: Already resolved by DC-075's system/health endpoint which
checks disk space, memory, service health, and incidents.

All 1540 tests pass.

dashcaddy-api/src/app.js

@@ -736,6 +736,12 @@ async function createApp() {
     ok(res, { metrics: metrics.getSummary() });
   });
 
+  // DC-097: Prometheus text-format endpoint for Grafana/Prometheus scraping
+  apiRouter.get('/metrics/prometheus', (req, res) => {
+    res.set('Content-Type', 'text/plain; version=0.0.4');
+    res.send(metrics.toPrometheus());
+  });
+
   // Mount at /api/v1 (canonical, single version)
   app.use('/api/v1', apiRouter);
 

dashcaddy-api/src/monitoring/metrics.js

@@ -110,6 +110,56 @@ class Metrics {
     this.requests = { total: 0, byStatus: {}, byMethod: {}, byPath: {} };
     this.errors = { total: 0, byType: {} };
   }
+
+  /**
+   * DC-097: Prometheus text-format export for /metrics/prometheus
+   * Returns standard Prometheus exposition format text.
+   */
+  toPrometheus() {
+    const uptimeSec = Math.floor((Date.now() - this.startTime) / 1000);
+    const mem = process.memoryUsage();
+    const lines = [];
+
+    lines.push('# HELP dashcaddy_uptime_seconds Server uptime in seconds');
+    lines.push('# TYPE dashcaddy_uptime_seconds counter');
+    lines.push(`dashcaddy_uptime_seconds ${uptimeSec}`);
+
+    lines.push('# HELP dashcaddy_requests_total Total HTTP requests');
+    lines.push('# TYPE dashcaddy_requests_total counter');
+    lines.push(`dashcaddy_requests_total ${this.requests.total}`);
+
+    for (const [status, count] of Object.entries(this.requests.byStatus || {})) {
+      lines.push(`dashcaddy_requests_by_status{status="${status}"} ${count}`);
+    }
+
+    for (const [method, count] of Object.entries(this.requests.byMethod || {})) {
+      lines.push(`dashcaddy_requests_by_method{method="${method}"} ${count}`);
+    }
+
+    lines.push('# HELP dashcaddy_errors_total Total errors');
+    lines.push('# TYPE dashcaddy_errors_total counter');
+    lines.push(`dashcaddy_errors_total ${this.errors.total}`);
+
+    lines.push('# HELP dashcaddy_containers_deployed Total containers deployed');
+    lines.push('# TYPE dashcaddy_containers_deployed counter');
+    lines.push(`dashcaddy_containers_deployed ${this.business.containersDeployed}`);
+
+    lines.push('# HELP dashcaddy_process_memory_heap_used_bytes Heap memory used');
+    lines.push('# TYPE dashcaddy_process_memory_heap_used_bytes gauge');
+    lines.push(`dashcaddy_process_memory_heap_used_bytes ${mem.heapUsed}`);
+
+    lines.push('# HELP dashcaddy_process_memory_heap_total_bytes Heap memory allocated');
+    lines.push('# TYPE dashcaddy_process_memory_heap_total_bytes gauge');
+    lines.push(`dashcaddy_process_memory_heap_total_bytes ${mem.heapTotal}`);
+
+    lines.push('# HELP dashcaddy_business_metric Business metrics');
+    lines.push('# TYPE dashcaddy_business_metric counter');
+    for (const [key, val] of Object.entries(this.business)) {
+      lines.push(`dashcaddy_business_metric{metric="${key}"} ${val}`);
+    }
+
+    return lines.join('\n') + '\n';
+  }
 }
 
 module.exports = new Metrics();

dashcaddy-api/src/utilities/middleware.js

@@ -439,6 +439,8 @@ module.exports = function configureMiddleware(app, {
     { path: '/api/v1/health-checks/status', exact: true, method: 'GET' },
     // DC-075: System health endpoint for external uptime monitoring (UptimeRobot, BetterStack)
     { path: '/api/v1/system/health',   exact: true,  method: 'GET' },
+    // DC-097: Prometheus metrics endpoint (scraped by Prometheus, no auth)
+    { path: '/api/v1/metrics/prometheus', exact: true, method: 'GET' },
     // System Overview widget on the dashboard — needs the flattened CPU/mem
     // data without going through auth. See skill references/totp-and-system-overview-pitfalls.md §3.
     { path: '/api/v1/monitoring/stats',      exact: true, method: 'GET' },