feat: make upstream timeouts configurable per-remote

Keep the dial/TLS/response-header timeouts as defaults, but allow each
remote to override them via upstream_dial_timeout / upstream_tls_timeout /
upstream_response_header_timeout (seconds; 0 = default). Clients are cached
by their timeout set so remotes sharing a configuration also share a
connection pool.

Refs #67

internal/database/postgres.go

@@ -124,6 +124,9 @@ 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';
+		ALTER TABLE remotes ADD COLUMN IF NOT EXISTS upstream_dial_timeout INTEGER DEFAULT 0;
+		ALTER TABLE remotes ADD COLUMN IF NOT EXISTS upstream_tls_timeout INTEGER DEFAULT 0;
+		ALTER TABLE remotes ADD COLUMN IF NOT EXISTS upstream_response_header_timeout INTEGER DEFAULT 0;
 
 		CREATE TABLE IF NOT EXISTS rpm_metadata (
 			id              BIGSERIAL PRIMARY KEY,