Group pipeline runs into Langfuse sessions, tagged by trigger source

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.

test/app/pipelines/RecommendationPipeline.test.ts

@@ -25,6 +25,7 @@ function makeTracer() {
   const tracer = {
     startTrace: vi.fn().mockReturnValue(trace),
     flush: vi.fn().mockResolvedValue(undefined),
+    runWithSession: vi.fn().mockImplementation((_session: unknown, fn: () => unknown) => fn()),
   } as unknown as Tracer;
   return tracer;
 }