Implement token expiration warnings (P3.9)

Add email warnings for Garmin token expiration at 14-day and 7-day thresholds.
When the garmin-sync cron job runs, it now checks each user's token expiry and
sends a warning email at exactly 14 days and 7 days before expiration.

Changes:
- Add sendTokenExpirationWarning() to email.ts with differentiated subject
  lines and urgency levels for 14-day vs 7-day warnings
- Integrate warning logic into garmin-sync cron route using daysUntilExpiry()
- Track warnings sent in sync response with new warningsSent counter
- Add 20 new tests (10 for email function, 10 for sync integration)

Test count: 517 → 537

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

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

@@ -4,8 +4,10 @@ import { NextResponse } from "next/server";
 
 import { getCycleDay, getPhase, getPhaseLimit } from "@/lib/cycle";
 import { getDecisionWithOverrides } from "@/lib/decision-engine";
+import { sendTokenExpirationWarning } from "@/lib/email";
 import { decrypt } from "@/lib/encryption";
 import {
+  daysUntilExpiry,
   fetchBodyBattery,
   fetchHrvStatus,
   fetchIntensityMinutes,
@@ -19,6 +21,7 @@ interface SyncResult {
   usersProcessed: number;
   errors: number;
   skippedExpired: number;
+  warningsSent: number;
   timestamp: string;
 }
 
@@ -36,6 +39,7 @@ export async function POST(request: Request) {
     usersProcessed: 0,
     errors: 0,
     skippedExpired: 0,
+    warningsSent: 0,
     timestamp: new Date().toISOString(),
   };
 
@@ -61,6 +65,17 @@ export async function POST(request: Request) {
         continue;
       }
 
+      // Check for token expiration warnings (exactly 14 or 7 days)
+      const daysRemaining = daysUntilExpiry(tokens);
+      if (daysRemaining === 14 || daysRemaining === 7) {
+        try {
+          await sendTokenExpirationWarning(user.email, daysRemaining);
+          result.warningsSent++;
+        } catch {
+          // Continue processing even if warning email fails
+        }
+      }
+
       // Decrypt OAuth2 token
       const oauth2Json = decrypt(user.garminOauth2Token);
       const oauth2Data = JSON.parse(oauth2Json);