[Feature] Align POST /routes contract with its upsert behaviour

[EnduranceCode]

Description

The POST /routes endpoint in RouteRestController.save() currently behaves as an upsert:

• If routeDTO.id() is null, it creates a new route and returns HTTP 201 (Created)
• If routeDTO.id() is not null, it updates the existing route and returns HTTP 200 (OK)

However, the RouteApi interface only documents a @ApiResponse(responseCode = "201"), implying a create-only operation. The OpenAPI contract does not reflect the update path or the possibility of a 200 response.

Files affected:

• endurancetrio-app/src/main/java/com/endurancetrio/app/tracker/api/RouteApi.java
• endurancetrio-app/src/main/java/com/endurancetrio/app/tracker/api/RouteRestController.java

Proposed approach:
Add a dedicated PUT /routes/{id} endpoint for updates and make POST /routes strictly create-only (always returning 201). This aligns with REST best practices and keeps the API contract clean and predictable.

Acceptance Criteria

• A dedicated PUT /routes/{id} endpoint is added for route updates
• POST /routes becomes create-only and always returns HTTP 201
• The RouteApi interface is updated to document both endpoints correctly
• The RouteRestController implementation is updated accordingly
• The RouteService interface and RouteServiceMain are updated if needed
• Tests cover both the create and update paths
• The project builds and all tests pass without errors