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/app/api/cron/garmin-sync/route.ts

@@ -13,6 +13,11 @@ import {
   fetchIntensityMinutes,
   isTokenExpired,
 } from "@/lib/garmin";
+import {
+  activeUsersGauge,
+  garminSyncDuration,
+  garminSyncTotal,
+} from "@/lib/metrics";
 import { createPocketBaseClient } from "@/lib/pocketbase";
 import type { GarminTokens, User } from "@/types";
 
@@ -34,6 +39,8 @@ export async function POST(request: Request) {
     return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
   }
 
+  const syncStartTime = Date.now();
+
   const result: SyncResult = {
     success: true,
     usersProcessed: 0,
@@ -129,10 +136,17 @@ export async function POST(request: Request) {
       });
 
       result.usersProcessed++;
+      garminSyncTotal.inc({ status: "success" });
     } catch {
       result.errors++;
+      garminSyncTotal.inc({ status: "failure" });
     }
   }
 
+  // Record sync duration and active users
+  const syncDurationSeconds = (Date.now() - syncStartTime) / 1000;
+  garminSyncDuration.observe(syncDurationSeconds);
+  activeUsersGauge.set(result.usersProcessed);
+
   return NextResponse.json(result);
 }