feat(core/migrate): import txAdmin players database

[FjamZoo]

Description

Add a MASTER-only POST /api/migrate endpoint that ingests a txAdmin playersDB.json and imports players, identifiers, notes, bans, warns and whitelist approvals. Matching is by full identifier, so the import is idempotent and safe to re-run.
The import runs in a re-exec'd child process the parent pipes the raw JSON in and reads the summary back so a multi-MB migration never blocks the core event loop. Logged as migrate.txadmin in the audit trail.

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Tests

• Tested in development mode
• Passes typecheck
• Builds & runs on Windows
• Builds & runs on Linux

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Additional Notes

I tried using a normal worker, however that wasn't quite feasible due to the fact that a .exe already bundles all of our modules and cant contain a worker on top of it.

So having the bun child process was the second best thing IMO.