feat(host,web): multi-GPU selection — GPU inventory + preference API, web-console GPU card

- new crate::gpu (compiled on all platforms so the OpenAPI doc stays
  platform-independent): DXGI / sysfs GPU inventory with reboot-stable ids
  (PCI vendor:device + occurrence — LUIDs are per-boot), persisted auto/manual
  preference (<config>/gpu-settings.json, atomic temp+rename with in-memory
  rollback), one selection with precedence console preference >
  PUNKTFUNK_RENDER_ADAPTER > max VRAM and graceful fallback when the preferred
  GPU is absent, plus a live "in use" record (RAII session guard wrapped around
  every encoder open_video returns)
- fix: windows_gpu_vendor derived the encoder backend from DXGI adapter 0
  instead of the selected render adapter — on a hybrid box (e.g. Intel iGPU at
  index 0 + NVIDIA dGPU) the backend could disagree with the GPU the capture
  ring / IddCx render pin sit on. The NVENC 4:4:4 probe now also runs on the
  selected adapter (was: OS default), the codec/4:4:4 probe caches are keyed
  per selected GPU (were process-lifetime OnceLocks), and an explicit
  PUNKTFUNK_ENCODER conflicting with the selected GPU's vendor warns up front
- mgmt API: GET /api/v1/gpus (inventory + mode + preferred + next-session
  selection with reason + in-use GPU/backend/session-count) and
  PUT /api/v1/gpus/preference (validates mode/gpu_id before writing);
  openapi.json regenerated; the vdisplay render pin now also engages for a
  console preference (not just the env pin)
- web console: GPU card on the Host page — list with vendor + VRAM,
  Automatic / Prefer controls, Preferred / Next session / "In use · backend"
  badges, missing-preferred-GPU warning and env-pin note; en + de messages
- Linux: a matched manual preference picks the VAAPI render node and the
  NVENC-vs-VAAPI auto choice; auto mode is exactly the previous behavior

Validated live on the hybrid laptop (RTX 3500 Ada + Intel Arc Pro, which
enumerates twice — the occurrence ids disambiguate): enumerate, prefer,
bad-id 400, restart persistence, auto-restore keeping the stored pick.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-host/src/encode/linux/vaapi.rs

@@ -50,10 +50,13 @@ const fn fourcc(a: u8, b: u8, c: u8, d: u8) -> u32 {
     (a as u32) | ((b as u32) << 8) | ((c as u32) << 16) | ((d as u32) << 24)
 }
 
-/// The render node a VAAPI/DRM device should open. `PUNKTFUNK_RENDER_NODE` pins it on a multi-GPU
-/// box; the default is correct on a single-GPU host.
+/// The render node a VAAPI/DRM device should open, from [`crate::gpu::linux_render_node`]: a
+/// matched web-console GPU preference pins it, else `PUNKTFUNK_RENDER_NODE`, else the single-GPU
+/// default.
 fn render_node() -> CString {
-    let p = std::env::var("PUNKTFUNK_RENDER_NODE").unwrap_or_else(|_| "/dev/dri/renderD128".into());
+    let p = crate::gpu::linux_render_node()
+        .to_string_lossy()
+        .into_owned();
     CString::new(p).unwrap_or_else(|_| CString::new("/dev/dri/renderD128").unwrap())
 }