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>

db/migrations/002_fix_usernames.sql

@@ -0,0 +1,22 @@
+-- One-time migration: backfill old username entries when users change display names.
+-- The write-time UPDATE in db.go AddLog() handles future changes automatically.
+-- Run once after deploying:  psql -d YOUR_DATABASE_URL -f db/migrations/002_fix_usernames.sql
+
+UPDATE distance_logs d
+SET username = (
+    SELECT username
+    FROM distance_logs sub
+    WHERE sub.user_id = d.user_id
+      AND sub.guild_id = d.guild_id
+    ORDER BY logged_at DESC
+    LIMIT 1
+)
+WHERE username <> (
+    SELECT username
+    FROM distance_logs sub
+    WHERE sub.user_id = d.user_id
+      AND sub.guild_id = d.guild_id
+    ORDER BY logged_at DESC
+    LIMIT 1
+);
+