fix: add logging to backend — analyze, job submit, status, cleanup

Backend had zero logging. Frontend logs were fine (logging.basicConfig
in app.py). Now both services write structured logs to systemd journal:

  journalctl -u vm-bench -f            # frontend (LXC)
  journalctl -u vm-bench-backend -f    # backend (srv2)

backend/app.py

@@ -8,7 +8,8 @@ Matches open-api.yaml v1.1.0 spec exactly.
 
 from __future__ import annotations
 
-from fastapi import FastAPI, HTTPException
+import logging
+from fastapi import FastAPI, HTTPException, Request
 from fastapi.middleware.cors import CORSMiddleware
 
 from models import (
@@ -24,6 +25,15 @@ from models import (
 from converter import extract_if_needed, discover_disk, detect_os, detect_efi
 from provisioner import submit_job, get_job_status, cleanup_staging
 
+# ---------------------------------------------------------------------------
+# Logging
+# ---------------------------------------------------------------------------
+logging.basicConfig(
+    level=logging.INFO,
+    format="%(asctime)s %(levelname)s %(message)s",
+)
+logger = logging.getLogger("backend")
+
 # ---------------------------------------------------------------------------
 # Constants
 # ---------------------------------------------------------------------------
@@ -71,20 +81,26 @@ def health() -> HealthResponse:
 def analyze(req: AnalyzeRequest) -> AnalyzeResponse:
     """Analyze a source file: extract if archive, find disk, detect OS + EFI."""
     _validate_filename(req.source_filename)
+    logger.info("Analyze request: vmid=%d file=%s", req.vmid, req.source_filename)
 
     try:
         source = extract_if_needed(req.source_filename)
+        logger.info("Source ready: %s", source)
     except Exception as exc:
+        logger.error("Extraction failed: %s", exc)
         raise HTTPException(status_code=400, detail=str(exc))
 
     try:
         disk = discover_disk(source)
+        logger.info("Disk found: %s (format=%s, size=%.1f GiB)", disk.path, disk.format, disk.size_gb)
     except Exception as exc:
+        logger.error("Disk discovery failed: %s", exc)
         raise HTTPException(status_code=400, detail=f"Disk discovery failed: {exc}")
 
     extract_dir = source if source.is_dir() else source.parent
     os_type = detect_os(disk.path, extract_dir)
     efi = detect_efi(disk.path)
+    logger.info("Analysis result: os=%s efi=%s", os_type, efi)
 
     return AnalyzeResponse(
         vmid=req.vmid,
@@ -99,9 +115,15 @@ def analyze(req: AnalyzeRequest) -> AnalyzeResponse:
 @app.post(f"{API_PREFIX}/jobs", response_model=JobStatusResponse, status_code=202)
 def create_job(req: JobSubmissionRequest) -> JobStatusResponse:
     """Submit a conversion + provisioning job."""
+    logger.info("Job submit: vmid=%d name=%s disk=%s storage=%s",
+                req.vmid, req.vm_name,
+                req.boot_disk.source_filename, req.target_storage)
     try:
-        return submit_job(req)
+        result = submit_job(req)
+        logger.info("Job queued: %s", result.job_id)
+        return result
     except Exception as exc:
+        logger.error("Job submission failed: %s", exc)
         raise HTTPException(status_code=400, detail=str(exc))
 
 
@@ -109,8 +131,10 @@ def create_job(req: JobSubmissionRequest) -> JobStatusResponse:
 def job_status(job_id: str) -> JobStatusResponse:
     """Get current status of a conversion job."""
     try:
-        return get_job_status(job_id)
+        result = get_job_status(job_id)
+        return result
     except Exception:
+        logger.warning("Job not found: %s", job_id)
         raise HTTPException(status_code=404, detail=f"Job not found: {job_id}")
 
 
@@ -121,6 +145,7 @@ def cleanup_job(job_id: str, req: CleanupRequest) -> CleanupResponse:
         vmid = int(job_id.split("_")[1])
     except (IndexError, ValueError):
         vmid = 0
+    logger.info("Cleanup: job=%s vmid=%d delete=%s", job_id, vmid, req.delete_staging_files)
     result = cleanup_staging(vmid, req.delete_staging_files)
     return CleanupResponse(
         job_id=result["job_id"],