server: validate the backend before binding the port

[CryptVenture]

Split out of #369 as requested: this PR is only the startup backend probe. The language contract gate, the ASR detail fields and the unload locking are separate PRs.

The problem

--backend defaults to cuda and nothing verified it at startup. On a host where that backend is not registered the server still bound its port and served the whole UI — health, model list, downloads — with an engine that could not run anything:

$ audiocpp_server --ui --backend cuda --port 8096
$ curl localhost:8096/health
{"status":"ok","backend":"cuda","models":0,"ui":true,"ui_management":false}

/health reports ok on an Apple-silicon machine with no CUDA device. The operator only finds out at the first model load or generate, by which time the failure looks like a model problem rather than a startup flag.

The change

main() initialises the requested backend and device once before constructing the server, then frees it. A missing backend or device now aborts startup with the message the engine would have raised later:

$ audiocpp_server --ui --backend cuda --port 8096
audiocpp_server failed: CUDA backend requested but it is not registered in this build
(available: MTL:0 "Apple M4 Max" [GPU], BLAS:0 "Accelerate" [ACCEL], CPU:0 "Apple M4 Max" [CPU])

The message already names the devices that are available, so the operator can correct --backend/--device immediately. The probe runs on a path that loads the backend libraries anyway and does not touch the request path.

Validation

Backend: Metal, Apple M4 Max, macOS 15.

Command	main	this branch
--ui --backend cuda	binds, /health → {"status":"ok","backend":"cuda"}	exits, CUDA backend requested but it is not registered in this build
--ui --backend metal	binds, /health → {"status":"ok","backend":"metal"}	binds, /health → {"status":"ok","backend":"metal"}

Build and tests:

cmake --build build/macos-metal-tests -j 12
ctest --test-dir build/macos-metal-tests        # 40/40

Scope

One file, app/server/main.cpp, 15 added lines. Startup only; no request, model or response behaviour changes.

[CryptVenture]

Closing for now to stay inside the 3-concurrent-PR policy (see the discussion on #422). Nothing is wrong with the change and CI is green on it; I will reopen it as review slots free.