fix: P2+P3 bug fixes, hardening, and cleanup (18 files)

Bug fixes:
- Add applyEnvOverrides to LoadFromBytes (M05)
- Set state=failed on compose-up failure in selfupdate (M16)
- Clamp usableMB to min 0 in memory check (M22)
- Remove "manual" schedule from triggerAllCrossBackups (M23)
- Add mmcblk device handling for partition paths (M21)
- Fix stripPartition for mmcblk devices (L25)
- Fix TruncateStr for UTF-8 and negative maxLen (L05/L06)
- Fix AllDone to return false for empty restore plans (L14)
- Fix PushOnce to return actual errors (L39)
- Restore pending events on save failure in DrainPendingEvents (M03)
- Add duplicate check in AddStoragePath (M04)
- Call CleanupTempMounts after drive scan (H13)
- Log SetStep save errors (M25)

Hardening:
- Guard scheduler Start() against double-start (M14)
- Acquire mutex in scheduler Stop() before reading cancel (L24)
- Cap log lines parameter to 10000 (L31)
- Require POST for logout (L32)
- Use sync.Once for Server.Close() (L49)
- Panic on crypto/rand.Read failure in setup CSRF (L40)
- Validate Bearer token against Hub API key in CSRF (H16 fix)
- Replace custom hasPrefix with strings.HasPrefix (L13)
- Replace simpleHash with crc32.ChecksumIEEE (L48)

Cleanup:
- Remove dead imageName function (L02)
- Remove dead detectHostIPViaRoute function (L03)
- Rename shadowed copy variable to cp (L07)
- Copy DefaultEnabledEvents in GetNotificationPrefs early return (L09)
- Update BUGHUNT.md with comprehensive audit results

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

controller/internal/backup/restore_scan.go

@@ -4,6 +4,7 @@ import (
 	"log"
 	"os"
 	"path/filepath"
+	"strings"
 	"sync"
 	"time"
 )
@@ -131,9 +132,13 @@ func (rp *RestorePlan) UpdateApp(name, status, errMsg string) {
 }
 
 // AllDone returns true if all apps are done/failed/skipped.
+// Returns false for empty plans (no apps to restore).
 func (rp *RestorePlan) AllDone() bool {
 	rp.mu.RLock()
 	defer rp.mu.RUnlock()
+	if len(rp.Apps) == 0 {
+		return false
+	}
 	for _, app := range rp.Apps {
 		if app.Status != "done" && app.Status != "failed" && app.Status != "skipped" {
 			return false
@@ -280,13 +285,10 @@ func hasUserData(rsyncBase string) bool {
 	}
 	for _, e := range entries {
 		name := e.Name()
-		if name != "_config" && name != "_db" && !hasPrefix(name, ".") {
+		if name != "_config" && name != "_db" && !strings.HasPrefix(name, ".") {
 			return true
 		}
 	}
 	return false
 }
 
-func hasPrefix(s, prefix string) bool {
-	return len(s) >= len(prefix) && s[:len(prefix)] == prefix
-}