feat: add resume mode and idempotent behavior to /simulate/trigger endpoint

BREAKING CHANGE: end_date is now required and cannot be null/empty

New Features:
- Resume mode: Set start_date to null to continue from last completed date per model
- Idempotent by default: Skip already-completed dates with replace_existing=false
- Per-model independence: Each model resumes from its own last completed date
- Cold start handling: If no data exists in resume mode, runs only end_date as single day

API Changes:
- start_date: Now optional (null enables resume mode)
- end_date: Now REQUIRED (cannot be null or empty string)
- replace_existing: New optional field (default: false for idempotent behavior)

Implementation:
- Added JobManager.get_last_completed_date_for_model() method
- Added JobManager.get_completed_model_dates() method
- Updated create_job() to support model_day_filter for selective task creation
- Fixed bug with start_date=None in price data checks

Documentation:
- Updated API_REFERENCE.md with complete examples and behavior matrix
- Updated QUICK_START.md with resume mode examples
- Updated docs/user-guide/using-the-api.md
- Added CHANGELOG_NEW_API.md with migration guide
- Updated all integration tests for new schema
- Updated client library examples (Python, TypeScript)

Migration:
- Old: {"start_date": "2025-01-16"}
- New: {"start_date": "2025-01-16", "end_date": "2025-01-16"}
- Resume: {"start_date": null, "end_date": "2025-01-31"}

See CHANGELOG_NEW_API.md for complete details.

QUICK_START.md

@@ -108,6 +108,7 @@ curl -X POST http://localhost:8080/simulate/trigger \
   -H "Content-Type: application/json" \
   -d '{
     "start_date": "2025-01-16",
+    "end_date": "2025-01-16",
     "models": ["gpt-4"]
   }'
 ```
@@ -119,12 +120,14 @@ curl -X POST http://localhost:8080/simulate/trigger \
   "job_id": "550e8400-e29b-41d4-a716-446655440000",
   "status": "pending",
   "total_model_days": 1,
-  "message": "Simulation job created with 1 trading dates"
+  "message": "Simulation job created with 1 model-day tasks"
 }
 ```
 
 **Save the `job_id`** - you'll need it to check status.
 
+**Note:** Both `start_date` and `end_date` are required. For a single day, set them to the same value. To simulate a range, use different dates (e.g., `"start_date": "2025-01-16", "end_date": "2025-01-20"`).
+
 ---
 
 ## Step 6: Monitor Progress
@@ -234,12 +237,32 @@ curl -X POST http://localhost:8080/simulate/trigger \
   -H "Content-Type: application/json" \
   -d '{
     "start_date": "2025-01-16",
+    "end_date": "2025-01-16",
     "models": ["gpt-4", "claude-3.7-sonnet"]
   }'
 ```
 
 **Note:** Models must be defined and enabled in `configs/default_config.json`.
 
+### Resume from Last Completed Date
+
+Continue simulations from where you left off (useful for daily automation):
+
+```bash
+curl -X POST http://localhost:8080/simulate/trigger \
+  -H "Content-Type: application/json" \
+  -d '{
+    "start_date": null,
+    "end_date": "2025-01-31",
+    "models": ["gpt-4"]
+  }'
+```
+
+This will:
+- Check the last completed date for each model
+- Resume from the next day after the last completed date
+- If no previous data exists, run only the `end_date` as a single day
+
 ### Query Specific Results
 
 ```bash