feat: ticket create - sub-server selector

- ticket-create.js: added 子服务器 dropdown (group/server from
  /polls/server-list, supports anonymous users), sends 'server' field
- tickets.js POST: accept server param, resolve 'group/server' or
  alias/server_name from server_groups, store into tickets.server_name
- polls.js: new public GET /server-list (group_name+server_name only,
  no auth required) for ticket create page
- verified: 13 checks (frontend/backend/db/column/syntax)

public/js/pages/ticket-create.js

@@ -41,6 +41,17 @@ const TicketCreate = {
       }).catch(() => {});
     }
 
+    // 子服务器列表(server_groups: 分组/子服)
+    this.servers = [];
+    API.get('/polls/server-list').then(list => {
+      this.servers = list || [];
+      const sel = container.querySelector('#server-select');
+      if (sel) {
+        sel.innerHTML = '<option value="">不指定</option>' + this.servers.map(g =>
+          `<option value="${U.escJs(g.group_name)}/${U.escJs(g.server_name)}">${U.esc(g.group_name)} / ${U.esc(g.server_name)}</option>`).join('');
+      }
+    }).catch(() => {});
+
     container.innerHTML = `
       ${hideTabs ? '' : `<div class="tab-nav" id="type-tabs">
         <button class="${isReport?'active':''}" data-type="report">举报</button>
@@ -58,6 +69,11 @@ const TicketCreate = {
               <input name="title" required placeholder="${isReport ? '如：玩家破坏建筑' : isAppeal ? '如：申请解除封禁' : '如：增加领地保护功能'}">
             </div>
 
+            <div class="form-group">
+              <label>子服务器</label>
+              <select name="server" id="server-select"><option value="">不指定</option></select>
+            </div>
+
             <div class="form-group">
               <label>优先级</label>
               <select name="priority">