feat(elections): admin voter roll CSV import and token generation

Summary

Let admins upload a voter roll CSV and generate one-time voting tokens for each voter.

Part of #1933.

Requirements

• CSV upload UI on the election detail page (columns: email, name)
• Validate CSV: unique emails, valid email format, row count limits
• Preview imported rows before committing
• Generate a cryptographically random token per voter, store only the salted hash
• Provide a "regenerate token" per-voter action (invalidates the previous hash, records audit entry)
• Provide a "revoke voter" action that invalidates the token without deletion (audit-preserving)
• Support appending additional voters to an already-imported roll
• Download a CSV of invited voters with status (invited/voted/revoked) — no tokens included
• All voter-roll mutations write to the audit log

Context

• Epic: #1933
• Depends on the Convex schema issue and admin UI issue

Implementation notes

• Tokens: 32 bytes of randomness, base64url-encoded → stored as sha256(salt + token) in `voterTokens.tokenHash`
• Plaintext token is only ever in the email that gets sent and the URL the voter clicks; server stores the hash
• Large imports should stream rows to avoid Convex mutation size limits