Add idempotent v2 vote endpoint

[ejones23]

Summary

• add typed anonymous POST /api/v2/votes.php envelopes and error states
• make retries safe with a ballot-scoped request key and payload hash
• enforce cutoff, ranking ownership, and one-device-one-vote rules server-side
• add the production/SQLite schema migration and contract coverage

Verification

• V2VoteTest: 9 tests
• full JavaScript and PHPUnit suites
• production Vite build

This intentionally returns typed blockers for name, secure-code, and grouping ballots; collecting those fields remains Phase 2.

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[DavidMoritz]

Excellent endpoint design, Emmanuel! The idempotency pattern here is rock-solid — the double-check on duplicate key insert for race conditions, hash_equals() for timing-safe comparison, and the clean typed error envelope are all exactly right. The 9 test cases cover the important edge cases thoroughly, and every query is properly parameterized.

The way you've structured the validation to collect all field errors before responding is a really nice touch for client ergonomics. This is production-quality API work. Thank you so much for this — your dedication to getting the details right is what makes this project better! 🎯