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.

models.js

@@ -20,7 +20,9 @@ const ticketSchema = new mongoose.Schema({
     claimerId: String,
     creatorId: String,
     parentCategoryId: String,
-    pendingDelete: { type: Boolean, default: false }
+    pendingDelete: { type: Boolean, default: false },
+    game: String,
+    closedAt: Date
 });
 ticketSchema.index({ status: 1, lastActivity: 1 });
 ticketSchema.index({ senderEmail: 1, status: 1 });
@@ -61,3 +63,29 @@ mongoose.model('StaffSignature', new mongoose.Schema({
   tagline: { type: String, default: '' },
   updatedAt: { type: Date, default: Date.now }
 }));
+
+const staffActionSchema = new mongoose.Schema({
+    staffId:       { type: String, required: true },
+    type:          { type: String, required: true },
+    tier:          { type: Number, default: 0 },
+    ticketType:    String,
+    priority:      String,
+    game:          String,
+    senderEmail:   String,
+    creatorId:     String,
+    gmailThreadId: String,
+    guildId:       String,
+    createdAt:     { type: Date, default: Date.now },
+
+    // close-only
+    closerType:    String,
+    resolverId:    String,
+    wasClaimed:    Boolean,
+
+    // transfer-only
+    fromId:        String,
+    toId:          String
+});
+staffActionSchema.index({ staffId: 1, createdAt: -1 });
+staffActionSchema.index({ gmailThreadId: 1, createdAt: 1 });
+mongoose.model('StaffAction', staffActionSchema);