fix: audit perbaikan NATS subject, worker spawn, frontend options, pipeline PDF output

Bug fixes:
1. NATS subject doubled prefix — gateway publish ke tools.tools.scan.*
   padahal workers subscribe ke tools.scan.*. Fix: pakai group() method
2. Worker subscriptions pake tokio::select! — cuma satu subscription
   yang diproses, sisanya di-cancel. Fix: ganti ke tokio::spawn + loop
3. WebSocket URL hardcoded ke localhost:3001 — gak jalan di prod.
   Fix: infer dari window.location atau env var
4. Frontend scan options hardcoded — user gak bisa atur OCR/enhance/format.
   Fix: interactive toggles, selects, slider
5. Frontend compress quality hardcoded — user gak bisa atur kualitas.
   Fix: quality range slider
6. Pipeline cuma output PNG — gak ada PDF/OCR.
   Fix: generate searchable PDF kalo tesseract feature enabled

Enhancements:
- Tool::group() method added — returns 'scan', 'image', 'pdf' dll
- Dockerfile: --features tesseract pas cargo build
- leptess OCR: proper API usage (set_image_from_mem, recognize, etc.)

Co-Authored-By: Kilo <kilo@kilo.ai>

backend/gateway/src/nats/publisher.rs

@@ -16,8 +16,8 @@ impl NatsPublisher {
 
     /// Publish a job to the appropriate NATS subject.
     pub async fn publish_job(nats: &Client, tool: &Tool, job: &Job) -> Result<(), NatsError> {
-        let prefix = tool.subject_prefix();
-        let subject = nats::job_subject(prefix, &job.id.to_string());
+        let group = tool.group();
+        let subject = nats::job_subject(group, &job.id.to_string());
         let payload = serde_json::to_vec(job)
             .map_err(|e| NatsError::Publish(e.to_string()))?;