refactor(host): single-source ChannelPolicy, space-signed file grants, whole-file registration

- all authorization rules (owner/creator, per-channel read/write grants, who may
  remove whom, delete rules, structure management) live in ChannelPolicy; a test
  fails if they are reimplemented elsewhere
- ChannelInfo carries what the caller may do (can_*), member and invitation lists
  say who can be removed; clients display it and never derive it
- the host no longer stores or calls the file service: it only signs short-lived
  tickets from space permissions (create/put/get/delete/domain), and keeps an opaque
  fileDomain reference on the channel
- a whole file is registered in one request (last frame header + per-frame length and
  digest) instead of one request per 1 MiB frame; the signed manifest still has to
  match every digest
- drop file quota/threshold, receipts, reconcile and delete-queue code from the host
- contract generator emits constants for enums, error codes, token roles, permissions
  and share states

server/WebDrop.slnx

@@ -1,10 +1,17 @@
 <Solution>
-  <Folder Name="/src/">
+  <!-- 主后端(hostAPI):账号、空间、频道、聊天、实时推送,以及内嵌的前端 -->
+  <Folder Name="/src/host/">
     <Project Path="src/WebDrop.Api/WebDrop.Api.csproj" />
-    <Project Path="src/WebDrop.Core/WebDrop.Core.csproj" />
+    <Project Path="src/WebDrop.Frontend/WebDrop.Frontend.csproj" />
+  </Folder>
+  <!-- 文件服务(fileAPI):另一个进程,只保存超过阈值的大文件 -->
+  <Folder Name="/src/files/">
     <Project Path="src/WebDrop.FileApi/WebDrop.FileApi.csproj" />
     <Project Path="src/WebDrop.FileProtocol/WebDrop.FileProtocol.csproj" />
-    <Project Path="src/WebDrop.Frontend/WebDrop.Frontend.csproj" />
+  </Folder>
+  <!-- 两边共用:业务规则、数据库、S3 备份 -->
+  <Folder Name="/src/shared/">
+    <Project Path="src/WebDrop.Core/WebDrop.Core.csproj" />
     <Project Path="src/WebDrop.Storage/WebDrop.Storage.csproj" />
     <Project Path="src/WebDrop.Sync/WebDrop.Sync.csproj" />
   </Folder>