feat: add local repository type with repo_type field

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

internal/api/v2/remotes.go

@@ -58,6 +58,17 @@ func (h *RemotesHandler) create(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, fmt.Sprintf("invalid package type: %q", remote.PackageType), http.StatusBadRequest)
 		return
 	}
+	if remote.RepoType == "" {
+		remote.RepoType = models.RepoTypeRemote
+	}
+	if !remote.RepoType.Valid() {
+		http.Error(w, fmt.Sprintf("invalid repo type: %q", remote.RepoType), http.StatusBadRequest)
+		return
+	}
+	if remote.RepoType == models.RepoTypeRemote && remote.BaseURL == "" {
+		http.Error(w, "base_url is required for remote repositories", http.StatusBadRequest)
+		return
+	}
 	if err := h.db.CreateRemote(r.Context(), &remote); err != nil {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return