Speed up daily reindex: skip empty JSON rewrites and upsert only changed posts.

Keep channel mtime stable when merge finds no new posts, and rebuild FTS only
for new/changed content so large daily channels stay cheap. Drop --reindex-all
from renew.sh for incremental runs.

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

mattermost-dl.py

@@ -624,6 +624,10 @@ def export_channel(d: Driver, channel: str, user_id_to_name: Dict[str, str], out
                 merged_posts.append(post)
                 seen_ids.add(post["id"])
                 new_count += 1
+        if new_count == 0:
+            # Keep mtime stable so mm_index can skip this channel file.
+            print(f"  no new posts, leaving '{output_filepath}' unchanged")
+            return
         merged_posts.sort(key=lambda p: p["created"])
         for i, post in enumerate(merged_posts):
             post["idx"] = i