feat(batch-agent): add Langfuse prompt management

- _get_system_prompt helper: fetches managed prompts from Langfuse
  with hardcoded fallback (same pattern as chat service)
- journey.py: journey_system prompt manageable via Langfuse
- agent_runner.py: batch_file_classifier, batch_processing,
  batch_cloud_processing prompts all manageable via Langfuse
- redis_consumer.py: link_prompt_to_trace for all three handlers

services/batch-agent/app/journey.py

@@ -26,6 +26,7 @@ from langchain_core.messages import AIMessage, HumanMessage, SystemMessage, Tool
 
 from app.agents.filesystem_agent import FILESYSTEM_TOOLS
 from app.llm import get_llm
+import app.tracing as tracing
 
 logger = logging.getLogger(__name__)
 
@@ -144,7 +145,10 @@ def _build_system_prompt(
         if existing_template
         else ""
     )
-    return _SYSTEM_PROMPT_TEMPLATE.format(
+    # Try Langfuse-managed prompt first, fall back to hardcoded template
+    managed = tracing.get_prompt("journey_system", fallback=None)
+    template = managed if managed is not None else _SYSTEM_PROMPT_TEMPLATE
+    return template.format(
         directory=directory,
         data_types=", ".join(data_types),
         template_start=_TEMPLATE_START,