feat(scripts): add redownload-model.sh to force-refresh a GGUF

The downloader's test -f guard never picks up an upstream re-upload of
an existing filename. The new script moves the old file aside, re-runs
the compose downloader, restores on failure, and restarts llama-server.

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

docs/operations.md

@@ -1,6 +1,6 @@
 # Operations
 
-> **Source of truth:** [`scripts/update.sh`](../scripts/update.sh), [`.env.example`](../.env.example), [`models.yml`](../models.yml), and the `speed-bench` service in [`docker-compose.yml`](../docker-compose.yml) with its [`speed-bench/Dockerfile`](../speed-bench/Dockerfile) and [`scripts/bench.sh`](../scripts/bench.sh). This page explains *what the script does and why*; the steps and defaults live in the source. If they disagree, the source wins.
+> **Source of truth:** [`scripts/update.sh`](../scripts/update.sh), [`scripts/redownload-model.sh`](../scripts/redownload-model.sh), [`.env.example`](../.env.example), [`models.yml`](../models.yml), and the `speed-bench` service in [`docker-compose.yml`](../docker-compose.yml) with its [`speed-bench/Dockerfile`](../speed-bench/Dockerfile) and [`scripts/bench.sh`](../scripts/bench.sh). This page explains *what the script does and why*; the steps and defaults live in the source. If they disagree, the source wins.
 
 The stack runs on the **server**. After any change to the repo, run `./scripts/update.sh` there: it is the one command that brings the running stack in sync. It's safe to re-run at any time, and it only recreates what changed.
 
@@ -45,6 +45,13 @@ One `downloader` service in the `tools` compose profile fetches every file liste
 - **Files live in the named `models` volume**, not a host bind-mount. The downloader runs as root because the named volume is created root-owned.
 - **It needs internet on each run**: it installs `curl` and `yq` from Alpine's package mirror before reading the list.
 
+### Forcing a model re-download
+
+The skip check means `update.sh` never picks up an upstream change to a file that's already on disk (for example, a re-uploaded GGUF with the same name). [`scripts/redownload-model.sh`](../scripts/redownload-model.sh) forces it: with no argument it refreshes `LLAMA_MODEL_FILE` from `.env`, or pass a `file` name from `models.yml` (for example the drafter). It reuses the compose downloader, so the URL still lives only in `models.yml`.
+
+- The old file is renamed to `<file>.old` and deleted only after the new download succeeds. If the download fails or is interrupted, the old file is put back. This needs room for both copies in the `models` volume at once.
+- It ends by restarting `llama-server`, because the running process keeps the old file mapped until it restarts.
+
 ## Speed bench
 
 `speed-bench` runs llama.cpp's [SPEED-Bench client](https://github.com/ggml-org/llama.cpp/blob/master/tools/server/bench/speed-bench/README.md) against the running `llama-server`. It measures decode speed (`avg_pred_t/s`), latency and draft acceptance per category. Use it to compare drafters and their flags with numbers instead of gut feel. It's in the `tools` profile, so `update.sh` never builds or runs it. The first run builds the image.