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.

services/configSchema.js

@@ -25,7 +25,7 @@ const ALLOWED_CONFIG_KEYS = new Set([
   'EMAIL_ESCALATED3_CHANNEL_ID', 'DISCORD_ESCALATED3_CHANNEL_ID',
   // Roles and staff
   'ROLE_ID_TO_PING', 'ADDITIONAL_STAFF_ROLES', 'BLACKLISTED_ROLES',
-  'ADMIN_ID',
+  'ADMIN_ID', 'STATS_ADMIN_IDS',
   // Channel IDs
   'TRANSCRIPT_CHANNEL_ID', 'LOGGING_CHANNEL_ID', 'DEBUGGING_CHANNEL_ID',
   // Messages and labels