Link every match to its referee via referee_id

[pawelMSoftware]

Summary

Adds a referee_id foreign key to every one of the 488 match records in data/matches/{year}.json, resolving each match to its head referee in data/referees.json (created in a prior milestone). This is a purely additive schema change: no existing field, value, or id was touched, and no referee name/code is duplicated inline — only the UUID. Closes #20.

Type of change

• Data correction (a value in data/ was wrong)
• New tournament / new records
• Documentation
• Test suite

(This PR adds a schema-wide FK, so it touches data/, its supporting docs, and its test file together — checked all three boxes above.)

Source

data/referees.json (the FK target) and docs/referees-match-mapping.md (the canonical match-to-referee assignment, same source used to build referees.json itself). No new research was performed — this PR is a mechanical join of the two already-verified sources.

Checklist

• One logical change per PR (see docs/CONTRIBUTING.md)
• composer test passes locally (106 passed, 16853 assertions)
• Record id values were not regenerated (UUIDs are permanent — see docs/CONVENTIONS.md)
• docs/CHANGELOG.md updated under "Unreleased" (required for any change to data/)
• docs/CONVENTIONS.md / docs/DATA_MODEL.md updated: DATA_MODEL.md's MATCH schema (table + ER diagram) documents the new referee_id field, the REFEREE ||--o{ MATCH relationship was added to both the entity diagram and the cross-entity example, and the referees.json entity note no longer says "not yet linked"; DATA_SOURCES.md updated; tests/MatchesTest.php updated (referee_id added to the required-fields-in-order check, plus a new referential-integrity test against data/referees.json).