feat: add local repository type with repo_type field (#49)

Introduces repo_type (remote/local) as a separate axis from package_type
so that any package type can be hosted locally. A terraform local repo
is package_type=terraform + repo_type=local.

- Remote model gains RepoType field (defaults to "remote")
- Database schema adds repo_type column with migration for existing DBs
- V1 proxy adds /api/v1/local/{name}/* route for serving local files
- V2 upload via PUT /api/v2/remotes/{name}/files/{ns}/{type}/{file}.zip
  validates filename matches terraform-provider-{type}_{ver}_{os}_{arch}.zip
  and returns 409 on duplicate (no overwrites)
- index.json and {version}.json are computed on-the-fly from uploaded zips
  rather than stored as separate files
- V2 create validates repo_type and requires base_url only for remotes

---------

Co-authored-by: Ben Vincent <ben@unkin.net>
Reviewed-on: #49

ui/src/api/types.ts

@@ -1,6 +1,7 @@
 export interface Remote {
   name: string;
   package_type: string;
+  repo_type: string;
   base_url: string;
   description: string;
   username?: string;