refactor(tag): use orElseThrow over Optional.get in findOrCreate (#730)

The lowest-id tie-break stream is guarded non-empty, so .get() never
throws — but the project bans Optional.get(). Switch to .orElseThrow()
for the project idiom. No behaviour change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/tag/TagService.java

@@ -68,7 +68,7 @@ public class TagService {
         if (exact.isPresent()) return exact.get();                          // exact-case wins (edit round-trip replays the stored name)
         List<Tag> caseInsensitive = tagRepository.findAllByNameIgnoreCase(cleanName);
         if (!caseInsensitive.isEmpty()) {
-            return caseInsensitive.stream().min(Comparator.comparing(Tag::getId)).get(); // deterministic tie-break by id — never throw
+            return caseInsensitive.stream().min(Comparator.comparing(Tag::getId)).orElseThrow(); // deterministic tie-break by id — list is non-empty, never throws
         }
         return tagRepository.save(Tag.builder().name(cleanName).build());   // create-when-absent (orphan tag: null sourceRef/parentId)
     }