Each pipeline invocation (runAsync/searchAsync) now gets a fresh
sessionId, wrapped via the new Tracer.runWithSession() (implemented
with @langfuse/tracing's propagateAttributes, since v5 has no direct
sessionId/tags option on startObservation) so it shows up in
Langfuse's Sessions view. The session is also tagged with what
triggered the run: "cron", "api-list", "api-trigger", "api-search".
Verified: full test suite (79/79) passing, tsc/eslint clean, real
docker compose build + container smoke test against /health.
