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

.env.example

@@ -9,7 +9,7 @@ BASE_URL=http://localhost:3000
 
 # Optional
 HOST=0.0.0.0
-PORT=3000
+PORT=8000
 
 # CORS — comma-separated allowed origins, or * for permissive (default: *)
 CORS_ORIGINS=*