refactor(api)!: 平台 id 收尾统一——migu→mg、apple→ap

补上最后两个多字母 id，至此 15 个平台对外 id 全部为两字母：
kw kg tx wy qs xm me qt lr mg bd jx bl 5s ap

**破坏性变更**：id 出现在所有 URL、/sources 返回以及 data/auth.json 的
凭据键里，咪咕与 Apple Music 已存的登录态会失效，需重新登录。

- sdk/index.ts：id 与 supportQuality 映射键（展示名 `咪咕音乐` /
  `Apple Music` 保持不动）
- sdk/types.ts：Source 联合类型
- src/routes.ts：/stream 的 enum ['qs','migu'] → ['qs','mg']，以及
  /url/mg、/stream/mg 等路径与 schema 描述
- src/http/migu-stream.ts、src/routes-login.ts、src/server.ts
- test/smoke-test.ts、README.md、docs/API.md 同步

有意未改：域名与 API 路径（music.migu.cn、music.apple.com、
amp-api.music.apple.com、/v1/catalog/cn/apple-curators/…）、
标识符（registerMiguStream、appleAmpGet、APPLE_AMP_API_URL 等）、
sdk/migu 与 sdk/apple 的目录与文件名、错误文案里的平台名。

验证：typecheck ✓、pnpm test 112/112 ✓；另起临时实例实证 /sources
（15/15 平台 id 与展示名全对）与 /login/status，旧 id 零残留。

sdk/migu/playlist.ts

@@ -31,7 +31,7 @@ const toPlaylist = (item: MiguPlaylistItem, categoryId?: string) => {
     playCount: parseInt(String(item.playNum ?? ''), 10) || 0,
     creator: firstNonEmpty(item.userName, item.ownerName),
     link: miguPlaylistLink(playlistID),
-    source: 'migu' as const,
+    source: 'mg' as const,
     ...(categoryId ? { categoryId } : {}),
   }
 }
@@ -41,7 +41,7 @@ export const searchPlaylist = async(keyword: string, page = 1, limit = 20) => {
   const body = await searchAll(keyword, 'songlist', page, limit)
   const result: MiguPlaylistItem[] = body?.songListResultData?.result ?? []
   const list = result.map(item => toPlaylist(item)).filter(Boolean)
-  return { list, source: 'migu' as const, page, limit, total: list.length }
+  return { list, source: 'mg' as const, page, limit, total: list.length }
 }
 
 /** 静态分类（上游 miguPlaylistCategoryKeywords） */
@@ -67,7 +67,7 @@ const CATEGORY_KEYWORDS = [
 ]
 
 export const getCategories = () => ({
-  source: 'migu' as const,
+  source: 'mg' as const,
   list: [
     { id: '', name: '全部', group: '全部' },
     ...CATEGORY_KEYWORDS.map(c => ({ id: c.name, name: c.name, group: c.group, hot: Boolean(c.hot) })),
@@ -77,7 +77,7 @@ export const getCategories = () => ({
 export const getCategoryPlaylists = async(categoryId: string, page = 1, limit = 20) => {
   const keyword = String(categoryId ?? '').trim() || '华语'
   const { list } = await searchPlaylist(keyword, page, limit)
-  return { list: list.map(p => toPlaylist(p as any, keyword)).filter(Boolean), source: 'migu' as const, page, limit }
+  return { list: list.map(p => toPlaylist(p as any, keyword)).filter(Boolean), source: 'mg' as const, page, limit }
 }
 
 /** 歌单曲目（每页 50 全量翻页 + 去重；上游 GetPlaylistSongs） */
@@ -123,7 +123,7 @@ export const getPlaylistDetail = async(id: string, page = 1, limit = 50) => {
   const { songs, total } = await fetchPlaylistSongs(playlistID)
   const start = (page - 1) * limit
   return {
-    source: 'migu' as const,
+    source: 'mg' as const,
     info: { name: playlistID, img: songs[0]?.img ?? '', desc: '', author: '' },
     list: songs.slice(start, start + limit),
     page,