Fix Invalid Date error in auth middleware

Add parseDate helper that safely returns null for empty/invalid date
strings from PocketBase. This prevents RangeError when pino logger
tries to serialize Invalid Date objects via toISOString().

- Make garminTokenExpiresAt and lastPeriodDate nullable in User type
- Filter garmin-sync cron to skip users without required dates
- Add test assertions for null date handling

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

src/app/api/calendar/[userId]/[token].ics/route.test.ts

@@ -27,7 +27,8 @@ vi.mock("@/lib/pocketbase", () => ({
               id: user.id,
               email: user.email,
               calendarToken: user.calendarToken,
-              lastPeriodDate: user.lastPeriodDate.toISOString(),
+              // biome-ignore lint/style/noNonNullAssertion: mock user has valid date
+              lastPeriodDate: user.lastPeriodDate!.toISOString(),
               cycleLength: user.cycleLength,
               garminConnected: user.garminConnected,
             };