[M1] Add PostgreSQL development environment

[rodrigolinhas]

Goal

Provide a reproducible PostgreSQL instance for MindSpace V2 development and tests.

Expected Scope

• PostgreSQL container configuration (Docker Compose).
• Persistent development volume configuration.
• Environment variable specifications.
• Safe example configuration (.env.example).
• Startup and operational documentation.
• Health/readiness configuration where useful.

Acceptance Criteria

• PostgreSQL can be started reproducibly from repository instructions.
• No credentials/secrets are committed.
• .env.example or equivalent documents required variables.
• Database data directories are not committed (.gitignore updated).
• PostgreSQL version is explicitly defined (e.g., PostgreSQL 16 Alpine).
• Connection can be verified from the V2 backend.
• Existing SQLite V1 database remains untouched.

Dependencies

Issue A (#6) may proceed independently, but integration requires Issue A.

Suggested PR

PR 2 — PostgreSQL + persistence + configuration (with Issues C and D)