feat(mgmt/web): surface the host.env encoder pin so a conflicting GPU choice isn't invisible

listGpus gains encoder_pin (PUNKTFUNK_ENCODER when it actually pins something —
unset/auto stay null), and the console's GPU card shows it: a muted note when
the pin is compatible, an amber warning naming the pinned vendor and the next
session's GPU when it contradicts the selection (the host overrides such a pin
at session open — without this field the selection just looked broken). Docs
updated to say the adapter wins.

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

api/openapi.json

@@ -10,7 +10,7 @@
       "name": "MIT OR Apache-2.0",
       "identifier": "MIT OR Apache-2.0"
     },
-    "version": "0.18.0"
+    "version": "0.19.2"
   },
   "paths": {
     "/api/v1/clients": {
@@ -4506,6 +4506,13 @@
               }
             ]
           },
+          "encoder_pin": {
+            "type": [
+              "string",
+              "null"
+            ],
+            "description": "`PUNKTFUNK_ENCODER` (the host.env encoder pin), when set to something other than `auto`\n(e.g. `qsv`, `nvenc`, `amf`, `software`). A pin whose vendor contradicts the selected\nGPU is overridden at session open — the adapter wins — so the console can warn that the\npin is stale rather than letting the selection look broken."
+          },
           "env_override": {
             "type": [
               "string",