Add prestart service for backend setup

[raphaelauv]

Add prestart service to compose.yml for backend initialization.

otherwise backend fail with

ERROR:  relation "user" does not exist at character 138

2026-09-29T16:14:27.394867476Z 2026-09-29 16:14:27.394 UTC [78] STATEMENT:  SELECT "user".email, "user".is_active, "user".is_superuser, "user".full_name, "user".id, "user".hashed_password, "user".created_at 

2026-09-29T16:14:27.394872616Z FROM "user" 

2026-09-29T16:14:27.394876012Z WHERE "user".email = $1::VARCHAR