MVP: Stabilize backend boot and health endpoint

[DerJanniku]

Goal

Stabilize backend startup and provide a reliable local run path for MVP development.

Scope

• Ensure backend compiles and boots in Docker
• Verify DB migration run on startup
• Expose and validate /health
• Document startup troubleshooting in README/docs

Acceptance Criteria

• docker compose up --build starts backend without crash
• GET /health returns 200 with { "ok": true }
• Startup logs clearly show DB connection + migration status
• README contains updated troubleshooting section

Notes

This is a blocker for end-to-end development and file upload flows.