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/api/routes/agent_setup.py

@@ -32,7 +32,7 @@ from langchain_core.messages import AIMessage, HumanMessage, SystemMessage, Tool
 
 from app.agents.filesystem_agent import FILESYSTEM_TOOLS
 from app.config.settings import settings
-from app.core.langfuse_client import extract_usage, get_langfuse, get_prompt_or_fallback
+from app.core.langfuse_client import compile_prompt, extract_usage, get_langfuse, get_prompt_or_fallback
 from app.core.llm import get_llm
 
 logger = logging.getLogger(__name__)
@@ -160,7 +160,9 @@ def _build_system_prompt(
     template, prompt_obj = get_prompt_or_fallback(
         "journey_system", _JOURNEY_SYSTEM_PROMPT
     )
-    compiled = template.format(
+    compiled = compile_prompt(
+        template,
+        prompt_obj,
         directory=directory,
         data_types=", ".join(data_types),
         template_start=_TEMPLATE_START,