feat(blender): Blender 5.2.2 LTS on fv-ml1 GPU 3, on demand (Prime)

Uses linuxserver/blender (Selkies Wayland desktop, NVENC), digest-pinned. The
container sees only GPU 3, has restart "no", and runs only while in use,
because GPU 3 is the reserve card for a full-size vLLM seat. Web desktop on
:3001 with basic auth (vault fv-ml1/blender-web-password). /work is on /tank
and is not backed up; /config lives under /opt/docker (restic).

Acceptance: Cycles finds the card on OptiX and CUDA (sm_120 kernels ship in the
build). The heavy self-test renders in 3.54 s on OptiX vs 22.71 s on CPU, a
functional check with n=1. Web auth answers 401 without credentials and with a
wrong password, and 200 with the right one.

servers/fv-ml1/README.md

@@ -224,6 +224,10 @@ eats into a future big seat's profiling margin. Small seats go on GPU 0, which h
 the most uncommitted headroom (its seats commit util 0.88; GPU 1 is at 0.975 and
 GPU 2 at 0.96).
 
+**GPU 3 on-demand tenant (Prime, 2026-09-27): `blender`** (`stacks/blender/`). It is up only
+while in use (`restart: "no"`, about 270 MiB when idle with the desktop running, 0 when down).
+The reserve still stands: whenever a full-size seat takes GPU 3, Blender stays down.
+
 **Retired:**
 - `llama-swap` (former GGUF multiplexer on :9292) — replaced by dedicated
   per-model seats (e.g. `llama-charrp`); no longer running.