feat(langfuse): propagate user_id and session_id to all traces

- Add hash_user_id() to SHA-256 hash user IDs before sending to Langfuse
- Add langfuse_context() helper wrapping propagate_attributes()
- deep_agent: extract session_id from _debug context, wrap all agent
  runs and classifier with langfuse_context(user_id, session_id)
- agent_runner: add session_id param, pass run_id as session for batch
- agent_setup: wrap journey LLM calls with langfuse_context
- Remove redundant metadata dicts (now handled by propagate_attributes)

app/core/langfuse_client.py

@@ -39,8 +39,10 @@ Linking a prompt to a generation::
 
 from __future__ import annotations
 
+import hashlib
 import logging
-from typing import Any
+from contextlib import contextmanager
+from typing import Any, Generator
 
 logger = logging.getLogger(__name__)
 
@@ -145,3 +147,44 @@ def extract_usage(response: Any) -> dict[str, int]:
         "output": int(meta.get("output_tokens", 0)),
         "total": int(meta.get("total_tokens", 0)),
     }
+
+
+def hash_user_id(user_id: str) -> str:
+    """Return a SHA-256 hash of *user_id* for use as Langfuse ``user_id``.
+
+    This avoids sending raw database UUIDs to external observability services
+    while still providing a stable, deterministic identifier for per-user
+    metrics in the Langfuse dashboard.
+    """
+    return hashlib.sha256(user_id.encode()).hexdigest()
+
+
+@contextmanager
+def langfuse_context(
+    user_id: str | None = None,
+    session_id: str | None = None,
+) -> Generator[None, None, None]:
+    """Propagate ``user_id`` (hashed) and ``session_id`` to all Langfuse observations.
+
+    No-op when Langfuse is not configured or parameters are empty.
+    """
+    lf = get_langfuse()
+    if lf is None or (not user_id and not session_id):
+        yield
+        return
+
+    try:
+        from langfuse import propagate_attributes
+    except ImportError:
+        logger.debug("langfuse: propagate_attributes not available — skipping context")
+        yield
+        return
+
+    attrs: dict[str, str] = {}
+    if user_id:
+        attrs["user_id"] = hash_user_id(user_id)
+    if session_id:
+        attrs["session_id"] = session_id
+
+    with propagate_attributes(**attrs):
+        yield