server: (router) add model management API (#23976)

* wip

* server: (router) add SSE realtime updates API

* nits

* wip

* add download API

* add download api

* update docs

* add delete endpoint

* fix std::terminate

* fix crash

* fix 2

* add tests

* nits

tools/server/README.md

@@ -1778,6 +1778,20 @@ The `status` object can be:
 }
 ```
 
+Note: for "downloading" state, there can be multiple files be downloading in parallel
+
+```json
+"status": {
+  "value": "downloading",
+  "progress": {
+    "https://...model.gguf": {
+      "done": 195963406,
+      "total": 219307424
+    }
+  }
+}
+```
+
 ### POST `/models/load`: Load a model
 
 Load a model
@@ -1820,6 +1834,107 @@ Response:
 }
 ```
 
+### GET `/models/sse`: Real-time events
+
+Example events:
+
+```js
+{
+  "model": "...",
+  "event": "model_status",
+  "data": {
+    "status": "loading"
+  }
+}
+
+{
+  "model": "...",
+  "event": "download_progress",
+  "data": {
+    // note: there can be multiple files being downloaded in parallel
+    "https://...model.gguf": {
+      "done": 195963406,
+      "total": 219307424
+    }
+  }
+}
+
+{
+  "model": "...",
+  "event": "download_finished",
+  "data": {
+    "status": "loading"
+  }
+}
+
+{
+  "model": "...",
+  "event": "model_remove"
+}
+
+// special event: reload of the list of all models
+{
+  "model": "*",
+  "event": "models_reload"
+}
+```
+
+### POST `/models`: Download new model
+
+Trigger a new download (non-blocking), the progress can be tracked via SSE endpoint `/models/sse`
+
+To cancel model downloading, send an event to `/models/unload`
+
+Download procedure:
+- Send POST request to `/models`
+- Subscribe to `/models/sse` for updates
+- On downloading completed, you will receive either `download_finished` or `download_failed` event
+- Call GET `/models` to trigger model list update. If the download success, you should see the new model in the list
+
+Payload:
+
+```json
+{
+  "model": "ggml-org/gemma-3-4b-it-GGUF:Q4_K_M",
+}
+```
+
+Response (download is started in the background):
+
+```json
+{
+  "success": true
+}
+```
+
+Response (error, cannot start the download):
+
+```json
+{
+  "error": {
+    "code": 400,
+    "message": "model validation failed, unable to download",
+    "type": "invalid_request_error"
+  }
+}
+```
+
+### DELETE `/models`: Delete a model from cache
+
+IMPORTANT: only model stored in cache can be deleted. You cannot delete models in a preset.
+
+Model name must be passed via query param: `?model={name}`
+
+If delete success, it will send an SSE event of type `model_remove`
+
+Response:
+
+```json
+{
+  "success": true
+}
+```
+
 ## API errors
 
 `llama-server` returns errors in the same format as OAI: https://github.com/openai/openai-openapi