Add full fleet management (Feature 13): database migration for fleet_vehicles,
fleet_maintenance_schedules, and fleet_maintenance_logs tables; REST API blueprint
with CRUD, scheduling, logging, alerts, and stats endpoints; frontend with tabbed
UI (vehicles grid, maintenance schedules, history, overdue alerts); sidebar nav entry.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>