Clean up workshop: update deps, simplify Dockerfile and docker-compose

- Dockerfile: use COPY --from uv image, uv python install 3.12, uv sync
  with pyproject.flink.toml, remove USER root and apt-get
- docker-compose: postgres 18, redpanda v25.3.9, remove container_name,
  hostname, extra_hosts; rename redpanda-1 -> redpanda
- Bump postgresql JDBC driver 42.7.4 -> 42.7.10
- Makefile: docker-compose -> docker compose
- README: replace em dashes, update code examples, pgcli -> uvx pgcli,
  python3 -> python, add transcript insights (watermarks, checkpointing,
  back pressure, Spark vs Flink, operational cost)