[Self-host] Persist Postgres data in docker-compose

[cursor]

Problem

Root docker-compose.yml has no volume. docker compose down / recreate wipes the local Postgres database.

Proposed change

Add a named volume on /var/lib/postgresql/data.

Why this matters

Local contributors lose orgs, API keys, and telemetry on a routine compose restart.

Suggested implementation

• docker-compose.yml
• One line in README or CONTRIBUTING noting that local Postgres data is persisted

Default compose should stay Postgres-only.

Acceptance criteria

• Postgres data survives container recreate
• Tests added/updated where applicable
• Documentation updated where applicable

Contributor notes

This should be a good first contribution and does not require deep knowledge of the Telemetry Tracker architecture.

Open PRs against develop. See CONTRIBUTING.md.