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/report/pusher.go

@@ -198,29 +198,27 @@ func (p *Pusher) PushOnce(report *Report) error {
 
 	data, err := json.Marshal(report)
 	if err != nil {
-		p.logger.Printf("[WARN] Hub report marshal failed: %v", err)
-		return nil
+		return fmt.Errorf("marshal report: %w", err)
 	}
 
 	url := p.hubURL + "/api/v1/report"
 
 	req, err := http.NewRequest(http.MethodPost, url, bytes.NewReader(data))
 	if err != nil {
-		return nil
+		return fmt.Errorf("create request: %w", err)
 	}
 	req.Header.Set("Content-Type", "application/json")
 	req.Header.Set("Authorization", "Bearer "+p.apiKey)
 
 	resp, err := p.httpClient.Do(req)
 	if err != nil {
-		p.logger.Printf("[WARN] Hub disabled-notification failed: %v", err)
-		return nil
+		return fmt.Errorf("hub push-once: %w", err)
 	}
 	io.Copy(io.Discard, resp.Body)
 	resp.Body.Close()
 
 	if resp.StatusCode >= 200 && resp.StatusCode < 300 {
-		p.logger.Printf("[INFO] Hub disabled-notification sent (%d bytes)", len(data))
+		p.logger.Printf("[INFO] Hub push-once sent (%d bytes)", len(data))
 	}
 	return nil
 }