feat(models): add Tag.display_name @property

Single source of truth for human-readable tag rendering. Falls back to
tag.name when no fandom is attached, preserving pre-migration data shapes.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/models.py

@@ -54,6 +54,19 @@ class Tag(db.Model):
         passive_deletes=True,
     )
 
+    @property
+    def display_name(self) -> str:
+        """Human-readable rendering of this tag.
+
+        After the bare-name refactor, `name` holds the display text directly for
+        every kind except character+fandom, which gets the '(Fandom)' suffix
+        appended here. Pre-migration data (with 'kind:' prefix in name) still
+        renders sanely via the else branch — callers see the raw stored string.
+        """
+        if self.kind == 'character' and self.fandom is not None:
+            return f"{self.name} ({self.fandom.name})"
+        return self.name
+
 class ArchiveRecord(db.Model):
     """
     Tracks archives we've processed so we can skip re-importing the same large file.