fix: prevent already-completed simulations from re-running

Previously, when re-running a job with some model-days already completed:
- _prepare_data() marked them as "skipped" with error="Already completed"
- But _execute_date() didn't check the skip list before launching executors
- ModelDayExecutor would start, change status to "running", and never complete
- Job would hang with status="running" and pending count > 0

Fixed by:
- _prepare_data() now returns completion_skips: {model: {dates}}
- _execute_date() receives completion_skips and filters out already-completed models
- Skipped model-days are not submitted to ThreadPoolExecutor
- Job completes correctly, skipped model-days remain with status="skipped"

This ensures idempotent job behavior - re-running a job only executes
model-days that haven't completed yet.

Fixes #73

api/simulation_worker.py

@@ -90,7 +90,7 @@ class SimulationWorker:
             logger.info(f"Starting job {self.job_id}: {len(date_range)} dates, {len(models)} models")
 
             # NEW: Prepare price data (download if needed)
-            available_dates, warnings = self._prepare_data(date_range, models, config_path)
+            available_dates, warnings, completion_skips = self._prepare_data(date_range, models, config_path)
 
             if not available_dates:
                 error_msg = "No trading dates available after price data preparation"
@@ -100,7 +100,7 @@ class SimulationWorker:
             # Execute available dates only
             for date in available_dates:
                 logger.info(f"Processing date {date} with {len(models)} models")
-                self._execute_date(date, models, config_path)
+                self._execute_date(date, models, config_path, completion_skips)
 
             # Job completed - determine final status
             progress = self.job_manager.get_job_progress(self.job_id)
@@ -145,7 +145,8 @@ class SimulationWorker:
                 "error": error_msg
             }
 
-    def _execute_date(self, date: str, models: List[str], config_path: str) -> None:
+    def _execute_date(self, date: str, models: List[str], config_path: str,
+                      completion_skips: Dict[str, Set[str]] = None) -> None:
         """
         Execute all models for a single date in parallel.
 
@@ -153,14 +154,24 @@ class SimulationWorker:
             date: Trading date (YYYY-MM-DD)
             models: List of model signatures to execute
             config_path: Path to configuration file
+            completion_skips: {model: {dates}} of already-completed model-days to skip
 
         Uses ThreadPoolExecutor to run all models concurrently for this date.
         Waits for all models to complete before returning.
+        Skips models that have already completed this date.
         """
+        if completion_skips is None:
+            completion_skips = {}
+
         with ThreadPoolExecutor(max_workers=self.max_workers) as executor:
             # Submit all model executions for this date
             futures = []
             for model in models:
+                # Skip if this model-day was already completed
+                if date in completion_skips.get(model, set()):
+                    logger.debug(f"Skipping {model} on {date} (already completed)")
+                    continue
+
                 future = executor.submit(
                     self._execute_model_day,
                     date,
@@ -397,7 +408,10 @@ class SimulationWorker:
             config_path: Path to configuration file
 
         Returns:
-            Tuple of (available_dates, warnings)
+            Tuple of (available_dates, warnings, completion_skips)
+            - available_dates: Dates to process
+            - warnings: Warning messages
+            - completion_skips: {model: {dates}} of already-completed model-days
         """
         from api.price_data_manager import PriceDataManager
 
@@ -456,7 +470,7 @@ class SimulationWorker:
         self.job_manager.update_job_status(self.job_id, "running")
         logger.info(f"Job {self.job_id}: Starting execution - {len(dates_to_process)} dates, {len(models)} models")
 
-        return dates_to_process, warnings
+        return dates_to_process, warnings, completion_skips
 
     def get_job_info(self) -> Dict[str, Any]:
         """