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/alerts.go

@@ -2,6 +2,7 @@ package web
 
 import (
 	"fmt"
+	"hash/crc32"
 	"log"
 	"strings"
 	"sync"
@@ -219,11 +220,7 @@ func (am *AlertManager) GetInlineAlerts(page string) []Alert {
 
 // simpleHash returns a short deterministic hash for deduplication.
 func simpleHash(s string) string {
-	h := uint32(0)
-	for _, c := range s {
-		h = h*31 + uint32(c)
-	}
-	return fmt.Sprintf("%08x", h)
+	return fmt.Sprintf("%08x", crc32.ChecksumIEEE([]byte(s)))
 }
 
 // sortAlerts sorts alerts by severity: error > warning > info.