fix(studio): review round 1 for imported-course editing

- Media exclusions move from the editable root .gitignore to the repo's
  info/exclude, and dirtyFiles never counts files listed in media.json, so an
  author's or remote's .gitignore cannot block publish or Sync. Publish and
  Sync refuse commits that track a file over referenced media.
- Importer jobs (code-repo, pdf-deck, transcribe) and the cover route run
  under the course lock now shared with the studio (core withCourseLock) and
  re-check the course inside it; after conversion they fail with "this course
  is now edited in the studio". Convert answers 409 while an import job for
  the course is queued or running, and the job re-checks.
- Crash-safe conversion: the import folder is hidden (.converting-<id>) before
  the published worktree exists; cross-filesystem moves copy into a staging
  dir, fsync, verify size and sha256, then rename; the source is retired as
  .converted-<id>.<n>. Recovery runs at server start and before every convert;
  a re-run of a finished conversion reports success. The loader skips
  dot-folders.
- Minor: no raw NUL bytes in source, link fallback only on EXDEV/EPERM/EMLINK,
  referenced media are not read whole by the editor, missing media are logged,
  mediaDir JSDoc, docs on per-author copies across filesystems.

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

packages/core/src/index.ts

@@ -3,3 +3,4 @@ export { createJobQueue } from "./jobs";
 export { serveMedia, contentType, contentDisposition, mediaSecurityHeaders, MEDIA_SANDBOX_CSP } from "./media";
 export { createApp, canView, crossSiteRefusal, type Routes } from "./app";
 export { composeServer, type ComposeOptions, type ComposedServer } from "./compose";
+export { withCourseLock } from "./locks";

packages/core/src/locks.ts

@@ -0,0 +1,14 @@
+const locks = new Map<string, Promise<unknown>>();
+
+/**
+ * Serialises work on one course across features in this process: the studio (saves, publish, conversion of an import)
+ * and the importer's jobs that add to an existing course. A failed holder never blocks the next one.
+ */
+export function withCourseLock<T>(courseId: string, fn: () => Promise<T>): Promise<T> {
+  const prev = locks.get(courseId) ?? Promise.resolve();
+  const next = prev.then(fn, fn);
+  const tail = next.catch(() => {});
+  locks.set(courseId, tail);
+  tail.then(() => { if (locks.get(courseId) === tail) locks.delete(courseId); });
+  return next;
+}