feat: implement date range API and on-demand downloads (WIP phase 2)

Phase 2 progress - API integration complete.

API Changes:
- Replace date_range (List[str]) with start_date/end_date (str)
- Add automatic end_date defaulting to start_date for single day
- Add date format validation
- Integrate PriceDataManager for on-demand downloads
- Add rate limit handling (trusts provider, no pre-config)
- Validate date ranges with configurable max days (MAX_SIMULATION_DAYS)

New Modules:
- api/date_utils.py - Date validation and expansion utilities
- scripts/migrate_price_data.py - Migration script for merged.jsonl

API Flow:
1. Validate date range (start <= end, max 30 days, not future)
2. Check missing price data coverage
3. Download missing data if AUTO_DOWNLOAD_PRICE_DATA=true
4. Priority-based download (maximize date completion)
5. Create job with available trading dates
6. Graceful handling of partial data (rate limits)

Configuration:
- AUTO_DOWNLOAD_PRICE_DATA (default: true)
- MAX_SIMULATION_DAYS (default: 30)
- No rate limit configuration needed

Still TODO:
- Update tools/price_tools.py to read from database
- Implement simulation run tracking
- Update .env.example
- Comprehensive testing
- Documentation updates

Breaking Changes:
- API request format changed (date_range -> start_date/end_date)
- This completes v0.3.0 preparation

api/date_utils.py

@@ -0,0 +1,93 @@
+"""
+Date range utilities for simulation date management.
+
+This module provides:
+- Date range expansion
+- Date range validation
+- Trading day detection
+"""
+
+import os
+from datetime import datetime, timedelta
+from typing import List
+
+
+def expand_date_range(start_date: str, end_date: str) -> List[str]:
+    """
+    Expand date range into list of all dates (inclusive).
+
+    Args:
+        start_date: Start date (YYYY-MM-DD)
+        end_date: End date (YYYY-MM-DD)
+
+    Returns:
+        Sorted list of dates in range
+
+    Raises:
+        ValueError: If dates are invalid or start > end
+    """
+    start = datetime.strptime(start_date, "%Y-%m-%d")
+    end = datetime.strptime(end_date, "%Y-%m-%d")
+
+    if start > end:
+        raise ValueError(f"start_date ({start_date}) must be <= end_date ({end_date})")
+
+    dates = []
+    current = start
+
+    while current <= end:
+        dates.append(current.strftime("%Y-%m-%d"))
+        current += timedelta(days=1)
+
+    return dates
+
+
+def validate_date_range(
+    start_date: str,
+    end_date: str,
+    max_days: int = 30
+) -> None:
+    """
+    Validate date range for simulation.
+
+    Args:
+        start_date: Start date (YYYY-MM-DD)
+        end_date: End date (YYYY-MM-DD)
+        max_days: Maximum allowed days in range
+
+    Raises:
+        ValueError: If validation fails
+    """
+    # Parse dates
+    try:
+        start = datetime.strptime(start_date, "%Y-%m-%d")
+        end = datetime.strptime(end_date, "%Y-%m-%d")
+    except ValueError as e:
+        raise ValueError(f"Invalid date format: {e}")
+
+    # Check order
+    if start > end:
+        raise ValueError(f"start_date ({start_date}) must be <= end_date ({end_date})")
+
+    # Check range size
+    days = (end - start).days + 1
+    if days > max_days:
+        raise ValueError(
+            f"Date range too large: {days} days (max: {max_days}). "
+            f"Reduce range or increase MAX_SIMULATION_DAYS."
+        )
+
+    # Check not in future
+    today = datetime.now().date()
+    if end.date() > today:
+        raise ValueError(f"end_date ({end_date}) cannot be in the future")
+
+
+def get_max_simulation_days() -> int:
+    """
+    Get maximum simulation days from environment.
+
+    Returns:
+        Maximum days allowed in simulation range
+    """
+    return int(os.getenv("MAX_SIMULATION_DAYS", "30"))