refactor: single-pass resolv.conf parsing, eliminate redundancies

Parse resolv.conf once for both upstream and search domains instead
of 2-3 reads. Extract CLOUD_VPC_RESOLVER constant. Use &'static str
for mode in StatsResponse. Remove dead read_upstream_from_file.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/api.rs

@@ -160,7 +160,7 @@ struct QueryLogResponse {
 struct StatsResponse {
     uptime_secs: u64,
     upstream: String,
-    mode: String,
+    mode: &'static str, // "recursive" or "forward" — never "auto" at runtime
     config_path: String,
     data_dir: String,
     dnssec: bool,
@@ -487,7 +487,7 @@ async fn stats(State(ctx): State<Arc<ServerCtx>>) -> Json<StatsResponse> {
     Json(StatsResponse {
         uptime_secs: snap.uptime_secs,
         upstream,
-        mode: ctx.upstream_mode.as_str().to_string(),
+        mode: ctx.upstream_mode.as_str(),
         config_path: ctx.config_path.clone(),
         data_dir: ctx.data_dir.to_string_lossy().to_string(),
         dnssec: ctx.dnssec_enabled,