refactor(models): merge downloaders into one, driven by models.yml

Replace the per-model downloader services with a single downloader
that reads file/url pairs from models.yml. Downloads now go to a
.part file first, so an interrupted run can't leave a truncated
model that later runs skip.

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

models.yml

@@ -0,0 +1,7 @@
+# Model files the downloader fetches into the models volume; see docs/operations.md#model-downloads
+# `file` must match the LLAMA_*_FILE value in .env that loads it.
+models:
+  - file: Qwen3.8-27B-UD-Q4_K_XL.gguf
+    url: https://huggingface.co/unsloth/Qwen3.8-27B-GGUF/resolve/main/Qwen3.8-27B-UD-Q4_K_XL.gguf
+  - file: Qwen3.8-27B-DFlash2-Q4_K_M.gguf
+    url: https://huggingface.co/z-lab/Qwen3.8-27B-DFlash2-GGUF/resolve/main/Qwen3.8-27B-DFlash2-Q4_K_M.gguf