Add workshop Langfuse seed flow and module 8 updates

[annabellscha]

Summary

• add a committed Langfuse workshop seed bundle plus import tooling for traces, observations, and scores
• add one-command workshop seeding via npm run langfuse:seed and document the maintainer flow
• update the module 8/9 learner and instructor materials plus the prompt/evaluator support code for the self-healing loop

Validation

• npm run typecheck
• live-tested the Langfuse seed import flow against the committed bundle during implementation