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

Dockerfile

@@ -52,7 +52,7 @@ WORKDIR /app
 COPY --from=builder /build/target/release/thoughts        ./thoughts
 COPY --from=builder /build/target/release/thoughts-worker ./thoughts-worker
 
-EXPOSE 3000
+EXPOSE 8000
 
 ENV RUST_LOG=info