feat: Add automated data validation script + GitHub Actions workflow

[sxryadipta]

Closes #57

What this PR adds

• scripts/validate_data.py — validates all JSON data files against the StatsBomb Data Specification v1.1
• .github/workflows/validate.yml — runs the script automatically on every push and weekly schedule
• Note: Tested on a shallow clone; the workflow runs successfully. Full data validation will execute against the complete dataset on merge.

Here are the checks covered:

Corruption & structure

• Null bytes / corrupted JSON files (fixes Corrupted JSON files in 360 events data with null bytes and malformed arrays #50)
• JSON parse errors

Match files

• Invalid competition_stage IDs
• Invalid/unknown data versions
• Bad date and time formats
• Negative scores
• Duplicate managers (fixes Check the data: Two managers are registered for the home team. #41)

Event files

• Coordinates out of pitch bounds, x:0–120 y:0–80 (fixes Goalkeepers event location invalid #49, Invalid locations for specific events: GoalKeeper, Pressure and ¿more? #34)
• Invalid period values (must be 1–5)
• Invalid play_pattern IDs
• team vs possession_team name mismatch (fixes Mismatches between the team names in "team" and "possession_team." #48)
• Carry events missing end_location
• Pass events missing length/angle/end_location
• Shot xG values outside 0–1
• Duel events missing type

Lineup files

• Duplicate player IDs within same team

Cross-file

• Match IDs with no corresponding events or lineups file

No dependencies
Pure Python standard library only, no pip installs needed.