feat(api): add POST /v1/images/upscale endpoint (#10227)

* feat(api): add POST /v1/images/upscale endpoint

Add a new image upscaling endpoint that accepts a source image and
returns an upscaled version. Supports selectable upscaler models
(e.g. realesrgan) and a configurable scale factor (2x or 4x).

- backend.proto: add UpscaleImage RPC and UpscaleImageRequest message
- pkg/grpc: implement UpscaleImage in Backend interface, client, server
  and embed shim
- core/backend/upscale.go: new backend helper (mirrors ImageGeneration)
- core/http/endpoints/openai/upscale.go: new multipart/form-data handler
- core/http/routes/openai.go: register POST /v1/images/upscale
- core/http/auth/features.go: gate upscale routes under FeatureImages
- backend/python/diffusers/backend.py: implement UpscaleImage — uses
  diffusers upscale pipeline when loaded, falls back to Lanczos resize

* fix(grpc): add UpscaleImage stub to Base backend

All Go backends embedding Base now satisfy the AIModel interface
without needing to implement UpscaleImage explicitly.

* fix(images): complete upscale endpoint integration

Store generated upscales under the served images directory, validate scale factors, document and advertise the endpoint, and add a functional Stable Diffusion x4 gallery model.

Assisted-by: Codex:gpt-5

---------

Co-authored-by: localai-org-maint-bot <306269227+localai-org-maint-bot@users.noreply.github.com>

backend/python/diffusers/backend.py

@@ -883,6 +883,34 @@ class BackendServicer(backend_pb2_grpc.BackendServicer):
 
         return backend_pb2.Result(message="Media generated", success=True)
 
+    def UpscaleImage(self, request, context):
+        try:
+            if not request.src:
+                return backend_pb2.Result(success=False, message="No source image provided")
+            if not request.dst:
+                return backend_pb2.Result(success=False, message="No destination path provided")
+
+            scale = request.scale if request.scale > 0 else 2
+            image = Image.open(request.src).convert("RGB")
+
+            # If the loaded pipeline supports upscaling (e.g. StableDiffusionUpscalePipeline),
+            # use it; otherwise fall back to high-quality Lanczos resize.
+            if self.pipe is not None and self.PipelineType in ("StableDiffusionUpscalePipeline", "StableDiffusionLatentUpscalePipeline"):
+                print(f"UpscaleImage: using diffusers upscale pipeline ({self.PipelineType})", file=sys.stderr)
+                upscaled = self.pipe(prompt="", image=image).images[0]
+            else:
+                # Fallback: high-quality Lanczos resize
+                print(f"UpscaleImage: no upscale pipeline loaded, using Lanczos resize (scale={scale})", file=sys.stderr)
+                new_w = image.width * scale
+                new_h = image.height * scale
+                upscaled = image.resize((new_w, new_h), Image.LANCZOS)
+
+            upscaled.save(request.dst)
+            return backend_pb2.Result(message="Image upscaled", success=True)
+        except Exception as e:
+            print(f"UpscaleImage error: {e}", file=sys.stderr)
+            return backend_pb2.Result(success=False, message=str(e))
+
     def GenerateVideo(self, request, context):
         try:
             prompt = request.prompt