docs: fix DX for new contributors

- Fix port mismatch: Dockerfile EXPOSE 8000, .env.example PORT=8000,
  compose.yml gets explicit PORT=8000
- Add thoughts-frontend/.env.example with all required vars
- Document NEXT_PUBLIC_FEDIVERSE_DOMAIN in README
- Document private cargo registry (k-ap on Gitea)
- Add local dev workflow: make dev-infra → cargo run → bun dev
- Split make targets: test-unit (no DB), test-integration, up

compose.yml

@@ -30,6 +30,7 @@ services:
       DATABASE_URL: postgres://postgres:postgres@postgres:5432/thoughts
       JWT_SECRET: change-me-in-production
       BASE_URL: http://localhost:8000
+      PORT: 8000
       NATS_URL: nats://nats:4222
       RUST_LOG: info
       STORAGE_BACKEND: local