LeetCode 901: Online Stock Span

## [LeetCode 901: Online Stock Span](https://leetcode.com/problems/online-stock-span/)

**Difficulty:** Medium
**Category:** Monotonic Stack

### Tasks
- [ ] Implement solution in `src/monotonic_stack/online_stock_span.py`
- [ ] Verify tests pass: `pytest tests/test_online_stock_span.py`
- [ ] Analyze time and space complexity