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.

.env.example

@@ -76,6 +76,7 @@ RATE_LIMIT_TICKETS_PER_USER=0     # Max tickets per user per window (0 = disable
 RATE_LIMIT_WINDOW_MINUTES=60      # Window in minutes for per-user rate limit
 BLACKLISTED_ROLES=                # Comma-separated role IDs that cannot open tickets
 ADDITIONAL_STAFF_ROLES=           # Comma-separated role IDs with staff permissions
+STATS_ADMIN_IDS=                  # Comma-separated Discord user IDs allowed to view other members' /stats (e.g. 321754640431710226,691678135527276614)
 
 # --- Auto-close ---
 AUTO_CLOSE_ENABLED=false