feat: add descriptive model naming and inline rename

- Auto-generate model names: {arch}-{labelType}-{epochs}ep-{classNames}-{YYYYMMDD}
- Add PATCH /api/models/{id}/rename endpoint
- Inline rename UI on Models & Training page
- Download filename uses model name instead of v{N}
- DB migration: add name column to model_versions
- Update all docs to reflect new naming convention

docs/requirements.md

@@ -336,7 +336,8 @@ changes.
   delta.
 - **REQ-064** — Each training run produces a stored model version (weights + metrics). The
   user can download the weights and **promote that version to be the project's new base
-  model** for the next round.
+  model** for the next round. Each version is auto-named
+  `{arch}-{labelType}-{epochs}ep-{classNames}-{YYYYMMDD}` and can be renamed by the user.
 - **REQ-065** — SAM3 and training must never hold VRAM at the same time; the system releases
   the SAM3 model before training starts.