Add next bookings endpoint

[jeromewir]

Summary

• add GET /api/bookings/next for upcoming WeWork bookings
• support optional startDate and endDate query params in YYYY-MM-DD format
• document endpoint usage and add date validation tests

Testing

• go test "api.go" "browser.go" "weworkrequests.go" "api_test.go" "weworkrequests_test.go" "dateformat.go" "auth.go"

Note: full go test ./... is blocked by pre-existing missing go.sum entries for untracked tracing.go OpenTelemetry imports.