Add per-staff metrics: StaffAction event log + /stats command

Event-sourced tracking of staff/ticket lifecycle actions, plus a /stats
command. Foundation for a future tickets-website analytics dashboard.

Data:
- StaffAction model (event log) + Ticket.game / Ticket.closedAt
- STATS_ADMIN_IDS config (who may view others' stats)

Recording (fire-and-forget, idempotent on real state transitions):
- claim, response (channel reply + /response send), escalate, de-escalate,
  transfer, close (4 sites), reopen — each denormalizes ticketType, tier,
  priority, game, requester (senderEmail / creatorId), guildId
- close events carry closerType / resolverId (claimer credit) / wasClaimed;
  transfer carries fromId / toId; reopen stamps resolverId
- conditional close transition helper (atomic open->closed + closedAt) shared
  by all four close paths

Query + command:
- pure period parser (presets + free-text) and stats shaper (per-metric keys)
- command-aware autocomplete dispatch
- /stats: period (autocomplete) + member (admin-gated) + source (all/email/
  discord), ManageMessages + staff-role gated, ephemeral, tier-labeled embed

288+ unit tests; timing/busiest-times data is collected but displayed later.

config.js

@@ -53,6 +53,7 @@ const CONFIG = {
   RATE_LIMIT_WINDOW_MINUTES: toInt(process.env.RATE_LIMIT_WINDOW_MINUTES, 60),
   BLACKLISTED_ROLES: (process.env.BLACKLISTED_ROLES || '').split(',').map(r => r.trim()).filter(Boolean),
   ADDITIONAL_STAFF_ROLES: (process.env.ADDITIONAL_STAFF_ROLES || '').split(',').map(r => r.trim()).filter(Boolean),
+  STATS_ADMIN_IDS: (process.env.STATS_ADMIN_IDS || '').split(',').map(r => r.trim()).filter(Boolean),
   TICKET_WELCOME_MESSAGE: process.env.TICKET_WELCOME_MESSAGE || "We got your ticket. We'll be with you as soon as possible. Feel free to add any additional information to your ticket.",
   TICKET_CLAIMED_MESSAGE: process.env.TICKET_CLAIMED_MESSAGE || 'Ticket claimed by {staff_mention} 🚀',
   TICKET_UNCLAIMED_MESSAGE: process.env.TICKET_UNCLAIMED_MESSAGE || 'Ticket unclaimed by {staff_mention} ☀️',