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/email.ts

@@ -2,6 +2,8 @@
 // ABOUTME: Sends daily training notifications and period confirmation emails.
 import { Resend } from "resend";
 
+import { emailSentTotal } from "@/lib/metrics";
+
 const resend = new Resend(process.env.RESEND_API_KEY);
 
 const EMAIL_FROM = process.env.EMAIL_FROM || "phaseflow@example.com";
@@ -57,6 +59,8 @@ Auto-generated by PhaseFlow`;
     subject,
     text: body,
   });
+
+  emailSentTotal.inc({ type: "daily" });
 }
 
 export async function sendPeriodConfirmationEmail(
@@ -114,4 +118,6 @@ Auto-generated by PhaseFlow`;
     subject,
     text: body,
   });
+
+  emailSentTotal.inc({ type: "warning" });
 }