Survive a first full crawl: poll, time out on silence, retry downloads

The first full crawl with the file manager ran 14 minutes, downloading every
file once, and broke in three ways:

- `schulcloud refresh` reported "fetch failed" for a crawl that was
  succeeding: Node's fetch abandons a response without headers after five
  minutes. POST /api/refresh takes wait:false and the CLI polls /api/status;
  refresh_index answers after 50 s and leaves the crawl running, and
  index_status says when a first crawl is under way.
- Downloads were bounded by the 30 s request timeout, which cut 11 MB scans
  off mid-transfer. They now time out on 30 s of silence instead.
- Failures were recorded once and never retried. A download failure is now
  retried on the next crawl while an extraction failure stays final, and PDF
  text containing NUL, which Postgres refuses, is stripped.

On the re-crawl all six failed files succeeded; only two videos above the
mirror cap stay metadata-only, by design. 137 tests.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/CLI.md

@@ -49,6 +49,10 @@ alongside courses, with their files under the room's name rather than a course's
 `refresh` asks the server to re-read Schulcloud. Pass `--course` when you know
 what changed: that is a handful of requests, where a full re-crawl reads every
 course. The server refuses a repeat within a minute unless you pass `--force`.
+A full re-crawl can take many minutes — the first one downloads every file,
+file-manager folders included — so `refresh` starts it and then polls the
+server's status, printing a note every half minute, rather than holding one
+request open (which Node's fetch abandons after five minutes).
 
 ### The file manager (`fs`)