Length and Duration as integer plus unit

[yepzdk]

Description

distance and totalDuration on Route are currently free-text strings (src/Entity/Route.php). Split each into a numeric value (integer) and a unit (dropdown/enum, e.g., meters/km for distance, minutes/hours for duration).

Implementation

• Add new fields (distanceValue, distanceUnit, durationValue, durationUnit) or use embedded value objects
• Create a Doctrine migration with data conversion for existing records
• Update admin form to use numeric input + unit dropdown
• Update API serialization to expose the structured data

Part of #35