Remove internal sd-server management, proxy to external sd-server

- Remove ProcessManager and process.py
- Add get_sd_server_url() config (env/config/default)
- Update routes to proxy to external sd-server URL
- Remove model switching (handled by external sd-server)
- Update CLI serve command
- Update tests for new architecture

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

uv.lock

@@ -707,7 +707,7 @@ wheels = [
 
 [[package]]
 name = "tensors"
-version = "0.1.9"
+version = "0.1.12"
 source = { editable = "." }
 dependencies = [
     { name = "httpx" },