Document or add Docker `docker` profile and DB init

[RonaldRonnie]

Description:

• Backend is run with SPRING_PROFILES_ACTIVE=docker in Docker Compose, but there is no application-docker.yml (or profile-specific config) in src/main/resources. Docker runs rely on env vars only; profile-specific defaults (e.g. Redis URL) are not documented.
• Original compose referenced ./config/docker/init.sql for Postgres; that file is missing. If schema or seed data are required, they are not applied in Docker.

Acceptance criteria:

• Add application-docker.yml (or document that Docker uses only env vars) and document required env vars
• If DB init is required: add config/docker/init.sql (or equivalent) and wire it in compose; otherwise remove or document the init mount