Implement Prometheus metrics endpoint (P2.16)

Add comprehensive metrics collection for production monitoring:
- src/lib/metrics.ts: prom-client based metrics library with custom counters,
  gauges, and histograms for Garmin sync, email, and decision engine
- GET /api/metrics: Prometheus-format endpoint for scraping
- Integration into garmin-sync cron: sync duration, success/failure counts,
  active users gauge
- Integration into email.ts: daily and warning email counters
- Integration into decision-engine.ts: decision type counters

Custom metrics implemented:
- phaseflow_garmin_sync_total (counter with status label)
- phaseflow_garmin_sync_duration_seconds (histogram)
- phaseflow_email_sent_total (counter with type label)
- phaseflow_decision_engine_calls_total (counter with decision label)
- phaseflow_active_users (gauge)

33 new tests (18 library + 15 route), bringing total to 586 tests.

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

src/lib/metrics.ts

@@ -0,0 +1,49 @@
+// ABOUTME: Prometheus metrics collection module for production monitoring.
+// ABOUTME: Exposes counters, gauges, and histograms for Garmin sync, email, and decision engine.
+
+import * as promClient from "prom-client";
+
+// Create a new registry for our application metrics
+export const metricsRegistry = new promClient.Registry();
+
+// Collect default Node.js metrics (heap, event loop, etc.)
+promClient.collectDefaultMetrics({ register: metricsRegistry });
+
+// Custom metric: Garmin sync operations counter
+export const garminSyncTotal = new promClient.Counter({
+  name: "phaseflow_garmin_sync_total",
+  help: "Total number of Garmin sync operations",
+  labelNames: ["status"] as const,
+  registers: [metricsRegistry],
+});
+
+// Custom metric: Garmin sync duration histogram
+export const garminSyncDuration = new promClient.Histogram({
+  name: "phaseflow_garmin_sync_duration_seconds",
+  help: "Duration of Garmin sync operations in seconds",
+  buckets: [0.1, 0.5, 1, 2, 5, 10],
+  registers: [metricsRegistry],
+});
+
+// Custom metric: Email sent counter
+export const emailSentTotal = new promClient.Counter({
+  name: "phaseflow_email_sent_total",
+  help: "Total number of emails sent",
+  labelNames: ["type"] as const,
+  registers: [metricsRegistry],
+});
+
+// Custom metric: Decision engine calls counter
+export const decisionEngineCallsTotal = new promClient.Counter({
+  name: "phaseflow_decision_engine_calls_total",
+  help: "Total number of decision engine calls",
+  labelNames: ["decision"] as const,
+  registers: [metricsRegistry],
+});
+
+// Custom metric: Active users gauge
+export const activeUsersGauge = new promClient.Gauge({
+  name: "phaseflow_active_users",
+  help: "Number of users with activity in the last 24 hours",
+  registers: [metricsRegistry],
+});