fix: deep bug hunt II — concurrency, security & optimization (25 files)

Critical: watchdog mutex panic safety, SetGeoAppOverride nil guard,
SSD-only app DB restore fallback.

High: double deploy race (atomic Deploying flag), delete/remove during
deploy guard, ScanStacks overwrite protection, FileBrowser mount mutex,
PushEvent history, PushOnce error handling, DB dump sync+close before
rename, restic retry fresh context, encrypt failure logging, cross-backup
path traversal validation, deepCopyStack completeness.

Security: constant-time API key comparison, login rate limiting (5/min),
git credential masking in logs, storage path prefix traversal fix.

Concurrency: MigrateEncryption lock ordering, SubdomainInUse I/O outside
lock, scheduler late-registered jobs, SQLite WAL verification, metrics
shutdown context, telemetry scan error logging, asset sync lock scope.

Optimization: streaming file copy for DB dumps, restic stats dedup,
atomic infra config copy.

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

controller/internal/backup/crossdrive.go

@@ -3,6 +3,7 @@ package backup
 import (
 	"context"
 	"fmt"
+	"io"
 	"log"
 	"os"
 	"os/exec"
@@ -475,12 +476,8 @@ func (r *CrossDriveRunner) copyStackDBDumps(stackName, destDir string) error {
 		}
 		src := filepath.Join(dumpDir, e.Name())
 		dst := filepath.Join(destDir, e.Name())
-		data, err := os.ReadFile(src)
-		if err != nil {
-			return fmt.Errorf("reading %s: %w", e.Name(), err)
-		}
-		if err := os.WriteFile(dst, data, 0644); err != nil {
-			return fmt.Errorf("writing %s: %w", e.Name(), err)
+		if err := copyFile(src, dst); err != nil {
+			return fmt.Errorf("copying %s: %w", e.Name(), err)
 		}
 		copied++
 	}
@@ -523,14 +520,11 @@ func (r *CrossDriveRunner) syncInfraConfig(ctx context.Context) {
 			}
 		}
 
-		// Copy controller.yaml → _infra/controller.yaml
+		// Copy controller.yaml → _infra/controller.yaml (atomic via copyFile)
 		if _, err := os.Stat(r.controllerYAMLPath); err == nil {
 			yamlDest := filepath.Join(infraDir, "controller.yaml")
-			data, err := os.ReadFile(r.controllerYAMLPath)
-			if err != nil {
-				r.logger.Printf("[WARN] Cannot read controller.yaml for infra backup: %v", err)
-			} else if err := os.WriteFile(yamlDest, data, 0644); err != nil {
-				r.logger.Printf("[WARN] Cannot write controller.yaml to %s: %v", yamlDest, err)
+			if err := copyFile(r.controllerYAMLPath, yamlDest); err != nil {
+				r.logger.Printf("[WARN] Cannot copy controller.yaml to %s: %v", yamlDest, err)
 			}
 		}
 
@@ -628,6 +622,32 @@ func (r *CrossDriveRunner) updateStatus(stackName, status, errMsg string, durati
 	})
 }
 
+// copyFile copies src to dst using buffered streaming I/O (no full-file memory allocation).
+func copyFile(src, dst string) error {
+	in, err := os.Open(src)
+	if err != nil {
+		return err
+	}
+	defer in.Close()
+
+	tmp := dst + ".tmp"
+	out, err := os.Create(tmp)
+	if err != nil {
+		return err
+	}
+
+	if _, err := io.Copy(out, in); err != nil {
+		out.Close()
+		os.Remove(tmp)
+		return err
+	}
+	if err := out.Close(); err != nil {
+		os.Remove(tmp)
+		return err
+	}
+	return os.Rename(tmp, dst)
+}
+
 // dirSizeBytes returns the total byte size of all files under path.
 // H7: Walk errors are now propagated instead of silently swallowed.
 func dirSizeBytes(path string) (int64, error) {