phase1-3: apply review fixes and docs alignment

Implement phase 1 and 2 review actions (DMARC UI CSS fixes, bounded DMARC ingest stdin,
rate-limit eviction hardening, settings/user handler cleanup, shared atomic/supervisor helpers)
and phase 3 documentation/changelog updates including roadmap and accepted-risk notes.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/postfix/write.go

@@ -1,47 +1,14 @@
 package postfix
 
 import (
-	"fmt"
 	"os"
-	"path/filepath"
+
+	"github.com/mixeme/selfpost/internal/atomicfile"
 )
 
 // writeFileAtomic writes data to path via a temp file in the same directory
 // followed by a rename, so a concurrent Postfix reload only ever sees the
-// complete old or new map, never a partial write (security.md). It mirrors the
-// same primitive used for the OpenDKIM tables.
+// complete old or new map, never a partial write (security.md).
 func writeFileAtomic(path string, data []byte, perm os.FileMode) error {
-	dir := filepath.Dir(path)
-	tmp, err := os.CreateTemp(dir, ".tmp-*")
-	if err != nil {
-		return fmt.Errorf("create temp in %s: %w", dir, err)
-	}
-	tmpName := tmp.Name()
-	cleanup := true
-	defer func() {
-		if cleanup {
-			_ = os.Remove(tmpName)
-		}
-	}()
-
-	if _, err := tmp.Write(data); err != nil {
-		tmp.Close()
-		return fmt.Errorf("write temp %s: %w", tmpName, err)
-	}
-	if err := tmp.Chmod(perm); err != nil {
-		tmp.Close()
-		return fmt.Errorf("chmod temp %s: %w", tmpName, err)
-	}
-	if err := tmp.Sync(); err != nil {
-		tmp.Close()
-		return fmt.Errorf("sync temp %s: %w", tmpName, err)
-	}
-	if err := tmp.Close(); err != nil {
-		return fmt.Errorf("close temp %s: %w", tmpName, err)
-	}
-	if err := os.Rename(tmpName, path); err != nil {
-		return fmt.Errorf("rename %s -> %s: %w", tmpName, path, err)
-	}
-	cleanup = false
-	return nil
+	return atomicfile.Write(path, data, perm)
 }