fix(studio): review round 2 - a conversion that fails after its commit recovers without a restart

- The convert job reloads the course on success and on failure, so learners
  get the intact import (rolled back) or the published studio course, never a
  course whose folder moved away.
- POST /convert on a course with an incomplete conversion (.converting-<id>,
  staging or retired folders) queues a job that recovers and finishes it
  instead of answering 409; the studio page shows "Conversion incomplete" with
  Retry for the owner.
- The 409 for pending import jobs names their kinds and ids; the dead withLock
  in git.ts is gone; the dot-folder discovery rule is documented in
  docs/operations.md and spec section 5.
- Route tests (SQLite and Postgres) inject a failed cross-filesystem copy, a
  checksum mismatch and a pre-commit failure.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/operations.md

@@ -143,6 +143,15 @@ Things to know:
 - After conversion the importer can no longer add decks, code or transcripts to that course (it only edits
   `<data>/courses`). Add them in the studio.
 - `<data>/media` is fixed under the data directory (there is no separate setting).
+- **Discovery skips dot-folders.** Every course directory (`STC_COURSE_DIRS`, `<data>/courses`, `<data>/worktrees`)
+  is scanned to depth 2 for `stcourse.yaml`, and folders whose name starts with `.` are never entered. Do not give a
+  course folder a name that starts with a dot. The conversion's `.converting-<id>` and `.converted-<id>.<n>` folders
+  rely on this.
+- **A failed conversion.** If the media move fails after the commit (disk full, or a copy that fails its checksum),
+  learners are switched to the studio course right away; its media are hard links into the hidden import folder.
+  The course's studio page shows **Conversion incomplete** with a **Retry** button for the owner. A retry (or the
+  next server start) finishes the move. A failure before the commit is rolled back, and learners keep the import
+  unchanged.
 
 ## Memory budget