refactor(transcription): extract rewriteBlockText from propagation loop

Extracts the Pattern+Matcher+replaceAll block into a private helper so the
loop body reads as three lines: rewrite text, update sidecar entries, nothing
else. Moves the boundary-condition rationale comment to the helper.

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

backend/src/main/java/org/raddatz/familienarchiv/service/PersonMentionPropagationListener.java

@@ -45,19 +45,12 @@ public class PersonMentionPropagationListener {
 
         String oldNeedle = "@" + event.oldDisplayName();
         String newNeedle = "@" + event.newDisplayName();
-        // Match @OldName only at a token boundary: not followed by a letter/digit/hyphen
-        // (catches @Hans-Peter when renaming Hans) AND not followed by " <Uppercase>"
-        // (catches @Hans Müller when renaming the single-name @Hans). False negatives —
-        // e.g. "@Hans Bekam" where Bekam is sentence-initial — are accepted as the
-        // conservative trade-off; the alternative (corruption) is irrecoverable.
         Pattern boundary = Pattern.compile(
                 Pattern.quote(oldNeedle) + "(?![\\p{L}0-9\\-]| (?=\\p{Lu}))");
         String replacement = Matcher.quoteReplacement(newNeedle);
 
         for (TranscriptionBlock block : blocks) {
-            if (block.getText() != null) {
-                block.setText(boundary.matcher(block.getText()).replaceAll(replacement));
-            }
+            rewriteBlockText(block, boundary, replacement);
             for (PersonMention mention : block.getMentionedPersons()) {
                 if (mention.getPersonId().equals(event.personId())) {
                     mention.setDisplayName(event.newDisplayName());
@@ -70,4 +63,15 @@ public class PersonMentionPropagationListener {
         log.info("Propagated rename {} → {} across {} block(s) for person {}",
                 event.oldDisplayName(), event.newDisplayName(), blocks.size(), event.personId());
     }
+
+    // Match @OldName only at a token boundary: not followed by a letter/digit/hyphen
+    // (catches @Hans-Peter when renaming Hans) AND not followed by " <Uppercase>"
+    // (catches @Hans Müller when renaming the single-name @Hans). False negatives —
+    // e.g. "@Hans Bekam" where Bekam is sentence-initial — are accepted as the
+    // conservative trade-off; the alternative (corruption) is irrecoverable.
+    private void rewriteBlockText(TranscriptionBlock block, Pattern boundary, String replacement) {
+        if (block.getText() != null) {
+            block.setText(boundary.matcher(block.getText()).replaceAll(replacement));
+        }
+    }
 }