feat: add admin password secret option

[bxn-dev]

Draft PR for adding Docker secret support for the initial OpenCloud admin password in the compose setup.

This adds:

• secrets/idm-admin-password.yml compose override
• .env.example hint for using the secret override
• README docs for IDM_ADMIN_PASSWORD_FILE

This depends on server support from:

• feat: support admin password file env opencloud#3034

Related Issue

• Fixes IDM_ADMIN_PASSWORD_FILE options in docker compose #306

Motivation and Context

Allows deployments to provide the IDM admin password via Docker secrets / mounted files instead of setting
INITIAL_ADMIN_PASSWORD directly in .env.

This PR should stay as draft until the related server PR is accepted, because IDM_ADMIN_PASSWORD_FILE needs server-side
support first.

How Has This Been Tested?

• test environment: local checkout on branch feat/306_idm_admin_password_file
• test case 1: default compose config still renders

docker compose config

• test case 2: secret override compose config renders

  printf '%s' secret > secrets/idm_admin_password
  COMPOSE_FILE=docker-compose.yml:secrets/idm-admin-password.yml docker compose config
  rm -f secrets/idm_admin_password

Screenshots (if appropriate):

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation added