Upgrade accounts saved as http:// to https on launch

Before the previous commit, browser sign-in could store an account as
http://, and the client now refuses to send to one. Left alone, such a
library would fail to open with a configuration error, so its stored
endpoint is rewritten before anything reads it.

The endpoint is half of two keys, and both are handled:

- The keyring entry is filed under it. Rewriting only the record would
  strand the app password under the old key and sign the user out, so
  AccountStore::move_endpoint copies the secret across first, rewrites
  the record in place (the last record is the one resumed), and deletes
  the old entry only once nothing refers to it.
- namespace() is built from it and names the catalog directory. For an
  http to https rewrite it does not change, because the namespace strips
  either scheme. A move that would change it is refused, not performed,
  so no later rewrite can abandon a catalog either.

The rewrite is a new BackendProvider::upgrade_endpoint hook, which does
nothing by default, and not a second call to normalise_endpoint. The
folder connector's normalise_endpoint canonicalises the path and needs
it to exist, so running it on every launch would fail a library on an
unplugged disk, or rename one whose path now resolves differently. Only
Nextcloud implements the hook.

If the move fails (for example, a locked keyring), it is logged, the
account is left as it was, and the move is tried again on the next
launch.

Closes #65.

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

@@ -96,6 +96,18 @@ impl BackendProvider for NextcloudProvider {
         }
     }
 
+    /// TRACES: NFR-SEC-3
+    /// An `http://` account written before sign-in kept the address the user
+    /// typed: it was stored as the server reported itself, and behind a proxy
+    /// without `overwriteprotocol` that is `http`. The client refuses to send
+    /// to it now, so it is upgraded here rather than left to fail. The
+    /// namespace ignores the scheme, so the catalog stays where it is.
+    fn upgrade_endpoint(&self, stored: &str) -> Option<String> {
+        stored
+            .strip_prefix("http://")
+            .map(|rest| format!("https://{rest}"))
+    }
+
     fn connect(&self, conn: &Connection) -> Result<Box<dyn RemoteBackend>, RemoteError> {
         let creds = Self::credentials(conn)?;
         Ok(Box::new(NextcloudBackend::new(
@@ -132,6 +144,18 @@ mod tests {
         assert!(p.normalise_endpoint("  ").is_err());
     }
 
+    /// TRACES: NFR-SEC-3
+    #[test]
+    fn a_stored_http_endpoint_is_upgraded_and_nothing_else_is_touched() {
+        let p = NextcloudProvider;
+        assert_eq!(
+            p.upgrade_endpoint("http://cloud.example/nextcloud")
+                .as_deref(),
+            Some("https://cloud.example/nextcloud")
+        );
+        assert_eq!(p.upgrade_endpoint("https://cloud.example"), None);
+    }
+
     #[test]
     fn the_account_keeps_the_dav_user_id_apart_from_the_login() {
         // A login can be an email address while the user id is something