feat(usenet): warn at startup when par2 or extractor is missing

A usenet-enabled agent silently produces corrupt files when par2 is
not installed: bad NNTP segments go unrepaired and unrar reports
checksum errors. Likewise, no unrar/7z means RAR-packed downloads
can't be unpacked at all.

When the registered agent has the usenet feature, check par2 and the
extractor (unrar/7z) in PATH and log a loud WARNING for each missing
one, mirroring the existing ffmpeg-for-HLS warning.

internal/agent/daemon.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"os/exec"
 	"runtime"
 	"strings"
 	"sync/atomic"
@@ -178,6 +179,21 @@ func (d *Daemon) Run(ctx context.Context) error {
 	log.Printf("Agent registered: %s (%s) [%s]", d.User.Name, d.User.Email, d.User.Plan)
 	log.Printf("Features: torrent=%v debrid=%v usenet=%v", d.Features.Torrent, d.Features.Debrid, d.Features.Usenet)
 
+	// Usenet needs par2 (segment repair) + an extractor (RAR/7z) on the host.
+	// Without par2, a single bad segment corrupts the file silently; without
+	// an extractor, RAR-packed downloads can't be unpacked. Warn loudly at
+	// startup so the operator installs them before the first download fails.
+	if d.Features.Usenet {
+		if _, err := exec.LookPath("par2"); err != nil {
+			log.Printf("[usenet] WARNING: par2 not found in PATH — corrupted segments cannot be repaired and extraction may fail. Install par2 (apt install par2 / brew install par2).")
+		}
+		_, unrarErr := exec.LookPath("unrar")
+		_, sevenZErr := exec.LookPath("7z")
+		if unrarErr != nil && sevenZErr != nil {
+			log.Printf("[usenet] WARNING: no archive extractor (unrar or 7z) found — RAR-packed downloads cannot be unpacked. Install unrar or 7z.")
+		}
+	}
+
 	// Wire sync callbacks
 	d.sync.OnNewTasks = func(tasks []Task) {
 		if d.OnTasksClaimed != nil {