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

thoughts-frontend/.env.example

@@ -0,0 +1,6 @@
+# API endpoints
+NEXT_PUBLIC_API_URL=http://localhost:8000              # client-side (browser) requests
+NEXT_PUBLIC_SERVER_SIDE_API_URL=http://localhost:8000   # SSR requests (use http://api:8000 inside Docker)
+
+# Fediverse handle display (optional)
+# NEXT_PUBLIC_FEDIVERSE_DOMAIN=yourinstance.example.com