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/agent_runner.py

@@ -45,7 +45,7 @@ from app.agents.task_agent import TASK_TOOLS
 from app.agents.timeline_agent import TIMELINE_TOOLS
 from app.config.settings import settings
 from app.core.device_manager import DeviceConnectionManager
-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
 from app.core.preprocessors import detect_content_type, preprocess
 from app.core.ws_context import clear_client_executor, execute_on_client, set_client_executor
@@ -661,7 +661,9 @@ async def run_local_agent(
                 )
                 metadata_section = _format_metadata(preprocessed.metadata)
 
-                system_prompt = unified_template.format(
+                system_prompt = compile_prompt(
+                    unified_template,
+                    prompt_obj,
                     filename=filename,
                     metadata_section=metadata_section,
                     projects_list=projects_block,
@@ -893,7 +895,9 @@ async def run_cloud_agent(
             cloud_template, cloud_prompt_obj = get_prompt_or_fallback(
                 "batch_cloud_processing", _BATCH_CLOUD_PROCESSING_PROMPT
             )
-            processing_prompt = cloud_template.format(
+            processing_prompt = compile_prompt(
+                cloud_template,
+                cloud_prompt_obj,
                 data_types=", ".join(config.data_types),
                 project_context="Determine the appropriate project from the message context.",
                 file_list=f"Message from {config.provider} (id: {msg.id})",