Adaption due to Understat dynamic data load

[JurekBauer]

Hi, this is the following PR. I heavily relied on LLMs, but worked for me fine. Up for a critical review:

Problem

Understat changed their website to load data dynamically via JavaScript, so the datesData and shotsData variables are no longer in the initial HTML. This caused understat-db ingest to fail with a ValueError when trying to extract JSON from script tags.

Solution
Switched to direct API endpoints instead of parsing HTML:
Matches: getLeagueData/{league}/{season} — returns JSON with dates, teams, and players
Shots: getMatchData/{match_id} — returns JSON with shots, rosters, and tmpl

Changes
Updated matches() to call the API endpoint directly
Updated shots() to call the API endpoint directly
Added required HTTP headers (Referer, User-Agent, Accept, X-Requested-With) for the API
Improved error handling in extract_json() with clearer messages
Kept fallback to the old HTML parsing method for backwards compatibility

Testing
✅ matches() works (tested with Bundesliga 2025)
✅ shots() works (tested with match ID 30224)
✅ understat-db ingest command now works correctly

Backwards Compatibility
The old HTML parsing method is still available as a fallback for cases where the API might not be accessible, ensuring backwards compatibility.