[Backend] Write `astre_server/.env.example` for Phase 1

[Muhammadcodes112]

Description

Document every environment variable Phase 1's backend actually reads, so a new contributor can get a local server running without guessing.

Where

astre_server/.env.example

What to implement

List (with a one-line comment each): DATABASE_URL, DB_HOST, DB_PORT, DB_USER, DB_PASSWORD, DB_NAME, PORT. Leave values blank or as obviously-fake placeholders (e.g. DB_PASSWORD=your-password-here) — never a real credential.

Acceptance Criteria

• Every env var actually read by Phase 1 backend code (check "Configure TypeOrmModule.forRoot...") appears here
• No var here is unused by Phase 1 code, and no var used by Phase 1 code is missing from here
• No real secrets, only placeholders

[sidd51]

I'd like to work on this, please assign it to me.