feat: Discord bridge adapter — 完整实现 Discord 作为第三个桥接渠道

新增文件：
- discord-adapter.ts: 基于 discord.js v14 的适配器，支持 DM/频道消息、
  按钮交互、流式预览(edit-based)、@提及过滤（兼容角色 mention）、
  图片附件下载、!→/ 命令归一化、消息去重、typing indicator
- markdown/discord.ts: Discord 2000 字符分段，代码块围栏自动平衡
- API routes: settings CRUD（Token 掩码）+ verify（Discord REST 验证）
- DiscordBridgeSection.tsx: 设置 UI（凭据、授权、服务器策略、配置指南含 ID 获取方法）
- discord-bridge.test.ts: 22 项单元测试（chunking/auth/command/HTML 转换）

修改文件：
- bridge-manager.ts: Discord 分段投递 + channel-aware stream config
  (Discord 默认 1500ms/40chars/1900max vs Telegram 700ms/20/3900)
- BridgeLayout/BridgeSection: Discord 渠道开关和侧栏入口
- bridge/settings/route.ts: 新增 14 个 Discord 设置键
- next.config.ts: serverExternalPackages 添加 discord.js/zlib-sync
- i18n: 中英文完整翻译（30+ keys）
- docs/handover/bridge-system.md: Discord 架构文档

依赖：discord.js ^14.18.0, zlib-sync ^0.1.9

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/api/bridge/settings/route.ts

@@ -22,6 +22,19 @@ const BRIDGE_SETTING_KEYS = [
   'bridge_feishu_group_policy',
   'bridge_feishu_group_allow_from',
   'bridge_feishu_require_mention',
+  'bridge_discord_enabled',
+  'bridge_discord_bot_token',
+  'bridge_discord_allowed_users',
+  'bridge_discord_allowed_channels',
+  'bridge_discord_allowed_guilds',
+  'bridge_discord_group_policy',
+  'bridge_discord_require_mention',
+  'bridge_discord_stream_enabled',
+  'bridge_discord_stream_interval_ms',
+  'bridge_discord_stream_min_delta_chars',
+  'bridge_discord_stream_max_chars',
+  'bridge_discord_max_attachment_size',
+  'bridge_discord_image_enabled',
 ] as const;
 
 export async function GET() {

src/app/api/settings/discord/route.ts

@@ -0,0 +1,73 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { getSetting, setSetting } from '@/lib/db';
+
+/**
+ * Discord Bot bridge settings.
+ * Stored in the SQLite settings table (same as other app settings).
+ */
+
+const DISCORD_KEYS = [
+  'bridge_discord_enabled',
+  'bridge_discord_bot_token',
+  'bridge_discord_allowed_users',
+  'bridge_discord_allowed_channels',
+  'bridge_discord_allowed_guilds',
+  'bridge_discord_group_policy',
+  'bridge_discord_require_mention',
+  'bridge_discord_stream_enabled',
+  'bridge_discord_stream_interval_ms',
+  'bridge_discord_stream_min_delta_chars',
+  'bridge_discord_stream_max_chars',
+  'bridge_discord_max_attachment_size',
+  'bridge_discord_image_enabled',
+] as const;
+
+export async function GET() {
+  try {
+    const result: Record<string, string> = {};
+    for (const key of DISCORD_KEYS) {
+      const value = getSetting(key);
+      if (value !== undefined) {
+        // Mask bot token for security (keep last 8 chars)
+        if (key === 'bridge_discord_bot_token' && value.length > 8) {
+          result[key] = '***' + value.slice(-8);
+        } else {
+          result[key] = value;
+        }
+      }
+    }
+
+    return NextResponse.json({ settings: result });
+  } catch (error) {
+    const message = error instanceof Error ? error.message : 'Failed to read Discord settings';
+    return NextResponse.json({ error: message }, { status: 500 });
+  }
+}
+
+export async function PUT(request: NextRequest) {
+  try {
+    const body = await request.json();
+    const { settings } = body;
+
+    if (!settings || typeof settings !== 'object') {
+      return NextResponse.json({ error: 'Invalid settings data' }, { status: 400 });
+    }
+
+    for (const [key, value] of Object.entries(settings)) {
+      if (!DISCORD_KEYS.includes(key as typeof DISCORD_KEYS[number])) continue;
+      const strValue = String(value ?? '').trim();
+
+      // Don't overwrite token if user sent the masked version back
+      if (key === 'bridge_discord_bot_token' && strValue.startsWith('***')) {
+        continue;
+      }
+
+      setSetting(key, strValue);
+    }
+
+    return NextResponse.json({ success: true });
+  } catch (error) {
+    const message = error instanceof Error ? error.message : 'Failed to save Discord settings';
+    return NextResponse.json({ error: message }, { status: 500 });
+  }
+}

src/app/api/settings/discord/verify/route.ts

@@ -0,0 +1,61 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { getSetting } from '@/lib/db';
+
+/**
+ * POST /api/settings/discord/verify
+ *
+ * Verifies Discord bot token by calling the Discord API /users/@me endpoint.
+ * If bot_token starts with "***" (masked), falls back to the stored token.
+ */
+export async function POST(request: NextRequest) {
+  try {
+    const body = await request.json();
+    let { bot_token } = body;
+
+    // Fall back to stored value if not provided or masked
+    if (!bot_token || bot_token.startsWith('***')) {
+      bot_token = getSetting('bridge_discord_bot_token') || '';
+    }
+
+    if (!bot_token) {
+      return NextResponse.json(
+        { verified: false, error: 'Bot token is required' },
+        { status: 400 },
+      );
+    }
+
+    // Call Discord API to verify the token
+    const res = await fetch('https://discord.com/api/v10/users/@me', {
+      method: 'GET',
+      headers: {
+        Authorization: `Bot ${bot_token}`,
+      },
+      signal: AbortSignal.timeout(10_000),
+    });
+
+    if (!res.ok) {
+      const errorData = await res.json().catch(() => ({}));
+      return NextResponse.json({
+        verified: false,
+        error: errorData.message || `HTTP ${res.status}: Token verification failed`,
+      });
+    }
+
+    const data = await res.json();
+
+    if (data.id) {
+      return NextResponse.json({
+        verified: true,
+        botName: data.username ? `${data.username}#${data.discriminator || '0'}` : data.id,
+      });
+    }
+
+    return NextResponse.json({
+      verified: false,
+      error: 'Could not retrieve bot info',
+    });
+  } catch (error) {
+    const message = error instanceof Error ? error.message : 'Verification failed';
+    return NextResponse.json({ verified: false, error: message }, { status: 500 });
+  }
+}