Upload a snapshot of a thumbnail shard, never the live file

Every shard is in WAL mode and every put opens its own connection, so
while thumbnails are being generated on several threads — which is when
the first sync pass runs — the log is never checkpointed and the main
file holds whatever the last quiet moment left in it. For a shard created
seconds earlier that is nothing: a zero-byte file with the schema still
in the log. The sync read that file and uploaded it, and every other
device merging it failed with "no such table: thumbs" on every pass.

Copy the shard through SQLite's backup API into scratch first, which
serialises against writers and carries the log, and upload that.

ui/dr-ui/src/derived_sync.rs

@@ -286,9 +286,20 @@ async fn sync_shards(
 
     // ---- upload ----------------------------------------------------------
     for shard in &local {
-        let path = store.shard_path(shard.id);
-        let Ok(bytes) = std::fs::read(&path) else {
-            continue;
+        // A snapshot, never the live file — see `ThumbStore::snapshot_shard`
+        // for the zero-byte upload that reading the file produced.
+        let snapshot = scratch.join(format!("thumb-shard-{:04}-upload.sqlite", shard.id));
+        let bytes = match store.snapshot_shard(shard.id, &snapshot) {
+            Ok(()) => std::fs::read(&snapshot),
+            Err(e) => Err(std::io::Error::other(e.to_string())),
+        };
+        let _ = std::fs::remove_file(&snapshot);
+        let bytes = match bytes {
+            Ok(b) => b,
+            Err(e) => {
+                log::warn!("snapshotting thumbnail shard {}: {e}", shard.id);
+                continue;
+            }
         };
         let name = shard_name(&client, shard.id);