fix(tag): warn when a tag's root cannot be resolved

resolveRoot silently falls back to returning the tag itself when no null-parent
ancestor surfaces — an orphaned parent_id or a chain deeper than the
findAncestorIds CTE depth guard. The chip then renders a non-root tag as if it
were the theme, with no trace. Log a warning (UUIDs only, per REQ-014) before
the fallback so the anomaly is diagnosable.

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

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

@@ -220,6 +220,11 @@ public class TagService {
                 if (ancestor != null && ancestor.getParentId() == null) return ancestor;
             }
         }
+        // No null-parent ancestor surfaced — the parent is orphaned or the chain is deeper than the
+        // findAncestorIds CTE's depth guard. Fall back to the tag as its own root, but surface it:
+        // a silently mislabeled root would otherwise be invisible. UUIDs only (no tag names logged).
+        log.warn("Tag {} has parent {} but no root surfaced from its ancestry; "
+                + "treating it as its own root.", tag.getId(), tag.getParentId());
         return tag;
     }