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/web/auth.go

@@ -128,6 +128,10 @@ func (s *Server) handleLogin(w http.ResponseWriter, r *http.Request) {
 }
 
 func (s *Server) handleLogout(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodPost {
+		http.Redirect(w, r, "/", http.StatusFound)
+		return
+	}
 	if cookie, err := r.Cookie(sessionCookieName); err == nil {
 		s.sessionsMu.Lock()
 		delete(s.sessions, cookie.Value)
@@ -203,9 +207,11 @@ func (s *Server) cleanupSessions() {
 	}
 }
 
-// Close signals the server to stop background goroutines.
+// Close signals the server to stop background goroutines. Safe to call multiple times.
 func (s *Server) Close() {
-	close(s.done)
+	s.closeOnce.Do(func() {
+		close(s.done)
+	})
 }
 
 func (s *Server) renderLogin(w http.ResponseWriter, errorMsg, flashMsg string) {