MSPCA-11 Add POST /recommendations endpoint

[Juwang110]

ℹ️ Issue

Closes MSPCA-11

📝 Description

Adds POST /api/recommendations, which lets a foster coordinator recommend a Chameleon animal to a volunteer before a formal match exists. The body is { volunteerId, chameleonAnimalId }, and the endpoint returns 200 with the saved recommendation (isActive: true). It returns 400 when either ID is missing or isn't a positive integer, and 404 when the volunteer doesn't exist.

Changes:

1. Added CreateRecommendationDTO, RecommendationsService.create, and the controller route. The route uses @HttpCode(200) because the ticket asks for 200 and Nest's default for POST is 201.
2. Added VolunteersService.existsById for the 404 check. RecommendationsModule now imports VolunteersModule, and it's registered in AppModule.
3. create upserts on the (volunteer_id, chameleon_animal_id) primary key. Recommending the same animal twice reactivates the existing row instead of failing with a PK violation, even when two coordinators do it at the same time.
4. validateId now uses Number.isInteger, so 1.5 and undefined are rejected.
5. VolunteersModule now imports CoordinatorsModule. Without it, registering FosterVolunteer through autoLoadEntities made the app fail on boot with "Entity metadata for FosterVolunteer#assignedCoordinator was not found".

✔️ Verification

• yarn test: all backend suites pass (20 suites, 160 tests). This covers new service and controller tests for success, the 404 case, each 400 case, and the 200 status code.
• yarn lint:check, yarn format:check, and tsc --noEmit on the backend are clean.
• To check by hand, start the backend and run:

  curl -i -X POST localhost:3000/api/recommendations \
    -H 'Content-Type: application/json' \
    -d '{"volunteerId": 1, "chameleonAnimalId": 42}'

  This should return 200 with isActive: true. A volunteer ID that doesn't exist should return 404, and a missing or non-integer ID should return 400.

🏕️ (Optional) Future Work / Notes