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.

src/core/langfuse/LangfuseTracer.ts

@@ -1,9 +1,15 @@
 import { NodeSDK } from '@opentelemetry/sdk-node';
 import { LangfuseSpanProcessor } from '@langfuse/otel';
-import { startObservation } from '@langfuse/tracing';
+import { propagateAttributes, startObservation } from '@langfuse/tracing';
 import type { LangfuseGeneration, LangfuseSpan } from '@langfuse/tracing';
 import { Tracer } from '../llm/Tracer';
-import type { GenerationHandle, GenerationInput, GenerationOutput, TraceHandle } from '../llm/Tracer';
+import type {
+  GenerationHandle,
+  GenerationInput,
+  GenerationOutput,
+  SessionInfo,
+  TraceHandle,
+} from '../llm/Tracer';
 
 export class LangfuseTracer extends Tracer {
   private readonly processor: LangfuseSpanProcessor;
@@ -48,6 +54,10 @@ export class LangfuseTracer extends Tracer {
     await this.processor.forceFlush();
   }
 
+  runWithSession<T>(session: SessionInfo, fn: () => Promise<T>): Promise<T> {
+    return propagateAttributes({ sessionId: session.sessionId, tags: session.tags }, fn);
+  }
+
   async shutdown(): Promise<void> {
     await this.sdk.shutdown();
   }