installer: reuse preserved .env/admin secret on reinstall and guard Postgres volume state

[ehsanking]

Motivation

• Improve reinstall workflow so preserved database credentials and admin bootstrap secret from installer backups are reused when available to avoid breaking existing DB state.
• Prevent accidental reinstall that would leave a persistent PostgreSQL volume with incompatible credentials and cause runtime failures.
• Make admin bootstrap behavior explicit for reinstall vs fresh installs so operator intent is clear and secure defaults are enforced.

Description

• Added helpers to compute the compose project name and PostgreSQL named volume (compose_project_name, postgres_named_volume, postgres_volume_exists).
• Added logic to capture environment variables from a live DB container (capture_reinstall_live_db_env) and to seed .env values from either live container env or the installer backup .env (seed_reinstall_env_from_backup).
• Restore the runtime admin bootstrap secret from the installer backup when present (restore_reinstall_admin_secret) and include the admin secret file in upgrade/reinstall backups.
• Added a guard that aborts reinstall when a persistent Postgres volume exists but compatible DB credentials were not restored (guard_reinstall_db_state).
• Adjusted configure_runtime_env to drive admin prompting/bootstrapping based on mode and presence of the DB volume, require ADMIN_USERNAME for fresh/reinstall, enforce restored admin password file when reusing runtime state, and set appropriate ADMIN_BOOTSTRAP_* flags for reinstall vs fresh scenarios.
• Wire capture_reinstall_live_db_env into main and report whether .env/admin secret were reused in the final summary.

Testing

• Ran bash -n install.sh to validate shell syntax and it completed successfully.
• Ran shellcheck install.sh static analysis and addressed reported issues (no blocking findings remained).

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