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/database/postgres.go

@@ -42,7 +42,8 @@ func (db *DB) migrate() error {
 		CREATE TABLE IF NOT EXISTS remotes (
 			name            TEXT PRIMARY KEY,
 			package_type    TEXT NOT NULL,
-			base_url        TEXT NOT NULL,
+			repo_type       TEXT DEFAULT 'remote',
+			base_url        TEXT NOT NULL DEFAULT '',
 			description     TEXT DEFAULT '',
 			username        TEXT DEFAULT '',
 			password        TEXT DEFAULT '',
@@ -121,6 +122,8 @@ func (db *DB) migrate() error {
 		);
 
 		CREATE INDEX IF NOT EXISTS idx_access_log_remote_time ON access_log(remote_name, created_at);
+
+		ALTER TABLE remotes ADD COLUMN IF NOT EXISTS repo_type TEXT DEFAULT 'remote';
 	`)
 	return err
 }