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>

package.json

@@ -21,6 +21,7 @@
     "node-cron": "^4.2.1",
     "pino": "^10.1.1",
     "pocketbase": "^0.26.5",
+    "prom-client": "^15.1.3",
     "react": "19.2.3",
     "react-dom": "19.2.3",
     "resend": "^6.7.0",