Ask the server about a collection once per job, not once per file

Every `move_to` guaranteed its destination's parent with a `MKCOL` for
each ancestor down from the account root, and a trash folder under a
library root several levels deep meant three round trips answering
`405 Method Not Allowed` before the one `MOVE` that did anything — for
every image of a delete, on a connection built for that job.

The backend now records the collections it has confirmed exist and asks
about each once. It lives for one job, so a folder another client removes
mid-batch is the one case this misses, and the `MOVE` then reports the
`409` rather than hiding it.

core/dr-sync-nextcloud/src/lib.rs

@@ -47,6 +47,20 @@ pub struct NextcloudBackend {
     /// `/remote.php/dav/files/<user>/` — the prefix stripped from hrefs.
     dav_base: String,
     caps: Capabilities,
+    /// Collections this backend has seen exist, so [`create_dir`] asks the
+    /// server about each one once.
+    ///
+    /// Every `MOVE` guarantees its destination's parent, and did so with a
+    /// `MKCOL` for each ancestor down from the account root — for a trash
+    /// folder three levels deep that was three round trips of `405 Method
+    /// Not Allowed` before the one request that moved anything, on every
+    /// image of a batch. A backend lives for one job (`dr_ui::remote::
+    /// connect` builds one per worker), so a folder deleted by another
+    /// client mid-job is the one case this can get wrong, and it is reported
+    /// as the `409` the `MOVE` then earns rather than hidden.
+    ///
+    /// [`create_dir`]: RemoteBackend::create_dir
+    known_dirs: std::sync::Mutex<std::collections::HashSet<String>>,
 }
 
 impl NextcloudBackend {
@@ -63,6 +77,7 @@ impl NextcloudBackend {
             login: creds.login_name.clone(),
             password: creds.app_password.clone(),
             dav_base,
+            known_dirs: Default::default(),
             caps: Capabilities {
                 // The property that makes a no-op sync one request (ARCH §8.1).
                 change_detection: ChangeDetection::PropagatingEtags,
@@ -567,6 +582,16 @@ impl RemoteBackend for NextcloudBackend {
         chain.reverse();
 
         for dir in chain {
+            // Asked once per backend — see `known_dirs`. The lock is held
+            // across no await: it is taken to look, and again to record.
+            let known = self
+                .known_dirs
+                .lock()
+                .map(|k| k.contains(dir.as_str()))
+                .unwrap_or(false);
+            if known {
+                continue;
+            }
             let url = self.url_for(&dir);
             let resp = self
                 .client
@@ -581,10 +606,12 @@ impl RemoteBackend for NextcloudBackend {
 
             // 405 is "already a collection here", which is exactly what the
             // caller wanted. Anything else is reported.
-            if resp.status() == reqwest::StatusCode::METHOD_NOT_ALLOWED {
-                continue;
+            if resp.status() != reqwest::StatusCode::METHOD_NOT_ALLOWED {
+                map_status(resp.status(), &url)?;
+            }
+            if let Ok(mut k) = self.known_dirs.lock() {
+                k.insert(dir.as_str().to_string());
             }
-            map_status(resp.status(), &url)?;
         }
         Ok(())
     }