Fix duplicate leaderboard entries, add /version command, fix jail DNS

- db/db.go: Add write-time username sync in AddLog to prevent duplicate

  leaderboard entries when users change display names. Revert correlated

  subqueries back to GROUP BY user_id, username (simpler approach).

- db/db.go: Early return in onMessageCreate if bot already reacted (prevents

  duplicate emoji reactions on Discord reconnection).

- bot/bot.go: Add /version slash command with build version injection.

- main.go: Add version variable with ldflags support.

- Makefile: Add dns-fix, test, vet, build-native, pg-*, boot targets.

  Prepend test+vet to deploy pipeline. Add version ldflags to build.

- db/migrations/002_fix_usernames.sql: One-time SQL to backfill old usernames.

- scripts/fix-jail-dns.sh: Script to update jail resolv.conf from 8.8.8.8

  to reachable nameservers (1.1.1.1, 9.9.9.9, 172.16.0.1).

Signed-off-by: Blake Ridgway <blake@blakeridgway.com>

cyclingboot

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