strip: remove unused CloseRequest mongoose model

Close-button confirmation uses the in-memory pendingCloses Map
(handlers/pendingCloses.js), not this persisted schema. Zero readers.

models.js

@@ -54,13 +54,6 @@ mongoose.model('Tag', new mongoose.Schema({
     useCount: { type: Number, default: 0 }
 }));
 
-mongoose.model('CloseRequest', new mongoose.Schema({
-    ticketId: { type: String, required: true, unique: true },
-    requestedBy: { type: String, required: true },
-    reason: String,
-    createdAt: { type: Date, required: true }
-}));
-
 mongoose.model('GuildSettings', new mongoose.Schema({
     guildId: { type: String, required: true, unique: true },
     emailRouting: { type: String, enum: ['thread', 'category'], default: 'category' },