feat: implement replace_existing parameter to allow re-running completed simulations

Add skip_completed parameter to JobManager.create_job() to control duplicate detection:
- When skip_completed=True (default), skips already-completed simulations (existing behavior)
- When skip_completed=False, includes ALL requested simulations regardless of completion status

API endpoint now uses request.replace_existing to control skip_completed parameter:
- replace_existing=false (default): skip_completed=True (skip duplicates)
- replace_existing=true: skip_completed=False (force re-run all simulations)

This allows users to force re-running completed simulations when needed.

api/main.py

@@ -284,7 +284,8 @@ def create_app(
                 config_path=config_path,
                 date_range=all_dates,
                 models=models_to_run,
-                model_day_filter=None  # Worker will filter based on available data
+                model_day_filter=None,  # Worker will filter based on available data
+                skip_completed=(not request.replace_existing)  # Skip if replace_existing=False
             )
             job_id = result["job_id"]
             warnings = result.get("warnings", [])