fix(langfuse): use compile() instead of .format() for prompt variable injection

Langfuse uses {{variable}} syntax in its prompt management UI, while the
hardcoded fallbacks use {variable} (Python str.format). The previous code
always called .format() which silently failed/errored when a real Langfuse
prompt was fetched.

- langfuse_client.py: add compile_prompt(template, prompt_obj, **vars)
  → uses prompt_obj.compile(**vars) when Langfuse is available
  → falls back to template.format(**vars) when using the hardcoded fallback
- agent_runner.py: replace .format() with compile_prompt() for
  unified_processing (V2 local) and batch_cloud_processing (cloud agent)
- agent_setup.py: replace .format() with compile_prompt() for journey_system

deep_agent.py prompts have no variables, so no change needed there.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/core/langfuse_client.py

@@ -80,10 +80,11 @@ def get_langfuse() -> Any | None:
 def get_prompt_or_fallback(name: str, fallback: str) -> tuple[str, Any]:
     """Fetch a text prompt from Langfuse; fall back to ``fallback`` on any error.
 
-    Returns ``(prompt_text, prompt_obj_or_None)``.
+    Returns ``(raw_template, prompt_obj_or_None)``.
 
-    * ``prompt_text`` — the raw template string (variables not yet substituted).
-      Callers perform variable substitution with Python's ``.format()``.
+    * ``raw_template`` — the uncompiled template string.  Do NOT call ``.format()``
+      on it directly; use :func:`compile_prompt` instead so the correct variable
+      syntax is applied (``{{var}}`` for Langfuse, ``{var}`` for the fallback).
     * ``prompt_obj`` — the Langfuse prompt object, or ``None`` when Langfuse is
       unavailable / the fetch failed.  Pass this to generation observations so
       Langfuse links the generation to the exact prompt version in the UI.
@@ -102,6 +103,38 @@ def get_prompt_or_fallback(name: str, fallback: str) -> tuple[str, Any]:
         return fallback, None
 
 
+def compile_prompt(template: str, prompt_obj: Any, **variables: Any) -> str:
+    """Compile *template* with *variables*, choosing the right syntax.
+
+    * When *prompt_obj* is a real Langfuse prompt object, calls
+      ``prompt_obj.compile(**variables)`` which handles ``{{variable}}``
+      substitution as defined in the Langfuse UI.
+    * When *prompt_obj* is ``None`` (Langfuse unavailable or fetch failed),
+      falls back to ``template.format(**variables)`` which handles the
+      ``{variable}`` syntax used in the hardcoded fallback strings.
+
+    This keeps callers oblivious to which syntax is in use.
+    """
+    if prompt_obj is not None:
+        try:
+            compiled = prompt_obj.compile(**variables)
+            # compile() returns a string for text prompts.
+            if isinstance(compiled, str):
+                return compiled
+            # Chat prompts return a list of dicts — join text parts.
+            if isinstance(compiled, list):
+                return "\n".join(
+                    m.get("content", "") for m in compiled if isinstance(m, dict)
+                )
+        except Exception as exc:
+            logger.warning(
+                "langfuse: compile failed for prompt %r: %s — falling back to .format()",
+                getattr(prompt_obj, "name", "?"),
+                exc,
+            )
+    return template.format(**variables)
+
+
 def extract_usage(response: Any) -> dict[str, int]:
     """Extract token usage from a LangChain AI message into Langfuse format."""
     meta = getattr(response, "usage_metadata", None)