refactor: one Range-capable file sender; HEAD support; assorted hygiene

web.py
* _download and _stream each had their own copy loop; _download had no
  Range support at all, so an interrupted download of a multi-hundred-MB
  recording had to restart. Both now go through _serve_bytes(), which
  keeps _stream's live-header `prefix` trick (the patched bytes are the
  same length as the header they replace, so Range math is unaffected).
* do_HEAD answers / , /stream/ and /download/ as a body-less do_GET.
  /api/clip and /api/cut return 405 instead — generating a clip or
  shelling out to ffmpeg only to discard the result is worse than not
  answering.
* _api_delete drops the deleted file's _DURATION_CACHE entry, which was
  never evicted.
* WAV durations are rounded to 2dp like FLAC's already were.

isr.py
* _open_output_file flushed the buffer and then called
  close_current_file(), which is overridden to flush again.
* log_level/log_file were merged into the dict copied into every
  source's config; no recorder reads them. Read them where they are used.

webui.html
* scoreOrder() is memoised on the section array's identity — U/I
  otherwise re-sorted a whole day (thousands of sections) per keypress.

Verified with a byte-level harness: full/partial/open-ended/unsatisfiable
ranges byte-identical to disk, download Range, HEAD on all three
supported paths returning headers with an empty body, 405 on /api/clip,
and live-header patching still correct both whole-file and for a range
straddling the patched/disk boundary.

Not done from the audit: BaseRecorder.generate_filename's mkdir per file
open. It runs once per split (once an hour by default), so "once at
startup suffices" was not worth a behaviour change. `nul` stays in
.gitignore — it guards against a Windows shell artifact on a Windows box.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -190,12 +190,14 @@ Everything the UI does goes through these endpoints, so they can also be scripte
 | `GET /api/clip?file=&start=&end=` | Section of a WAV/FLAC decoded server-side, returned as a standalone WAV (max 600 s) |
 | `GET /api/cut?file=&start=&end=` | ffmpeg-trimmed copy of the file as a download |
 | `GET /stream/<name>` | Inline playback with HTTP Range support; live files get an on-the-fly patched header |
-| `GET /download/<name>` | Raw file download |
+| `GET /download/<name>` | Raw file download, with HTTP Range support so an interrupted transfer can resume |
 | `GET /api/status` | Currently recording files (`status.json` passthrough) |
 | `GET /api/storage` | Disk free/total |
 | `GET /api/config` | Server-side defaults for margin, min-gap, and min-duration (seeds the UI controls) |
 | `DELETE /api/files/<name>` | Delete a recording and its analysis cache |
 
+`HEAD` is answered for `/`, `/stream/<name>` and `/download/<name>` (headers only, no body) — the generated endpoints (`/api/clip`, `/api/cut`) return `405` rather than doing the work and discarding it.
+
 Analysis, clips, cut, and delete return `409` for files that are still being recorded.
 
 ---