feat(route): 阶段 1 多渠道路由分发层 (Channel + Router)

- store: 新增 channels 表 (name unique, credentials_json, voices CSV,
  priority, weight, status, auto_ban) + CRUD (List/Get/Insert/SetStatus/Delete)
  + ErrChannelDuplicate 区分 (避免与 ErrDuplicate 互相误报)
- store/db.go: schemaVersion 1 -> 2; channels 表 CREATE IF NOT EXISTS + 复合索引
- adapter/route: 新包,Channel (含 BuildRequest 覆盖 template.Credentials)
  + Router (Select / SelectAndSynthesize / pickByWeight 纯函数)
  + 错误区分 ErrNoChannels (走兜底) vs ErrVoiceNotFound (400)
  + AggregateError 聚合多渠道失败, 供 controller 日志逐个打印
- controller/router.go: SetRouter/GetRouter 句柄 (与 SetAdminStore 同一模式)
- controller/tts.go: 接入 Router, 零 channel 走 setting 兜底 (行为=阶段 0);
  voice 不被任何 channel 接受 -> 400 + voice_not_found; 抽出 finalizeSynth
  收敛合成结果->响应+metrics+日志三件套
- main.go: 启动期从 store 加载 channels, 转 route.Channel, 注入 controller;
  credentials_json 加载失败 -> fail-fast (避免一个错渠道拖崩全部请求)
- .gitignore: 补 .gotmp/ .gomodcache/ .dsh-acl/

验收 (设计文档 §9):
  - 零 channel 兜底, 行为=阶段 0
  - 加 1 个 channel: 走该渠道
  - 加 2+ 个同档不同 weight: pickByWeight weight-1-to-3 分布 ±5% (单测覆盖)
  - 不同 priority: 高优先级优先 (单测覆盖)
  - disabled 不参与选择 (单测覆盖)
  - 失败降级: 同档换下一个 -> 降档, 全失败返 AggregateError (单测覆盖)
  - go build ./... + go vet ./... 全绿
  - go test ./... (adapter/route 15 个, store 2 个, 全部通过)

.gitignore

@@ -1,5 +1,7 @@
 # Go build cache
 .gocache/
+.gotmp/
+.gomodcache/
 *.exe
 *.test
 *.out
@@ -49,3 +51,6 @@ tts.db
 tts.db-*
 tts.db.*
 installed.lock
+
+# DSH sandbox ACL repair artifacts (created by diagnose-windows-sandbox-acl skill)
+.dsh-acl/