Declare the AI owner, and let every node borrow its model over the mesh

vv_is_ai_host() was `=== 'host1'`, which made a physical fact — that is where the
GPU is — look like a rule. AI_OWNER_HOST declares it, so the card can move to a
rebuilt host3 or a friend's spare.

The gate was also doing two jobs. Assistant docks and findings strips now ask
whether a model is reachable, so a node without a GPU gets them by borrowing;
the AI tab asks whether this is the owner, because that page carries the bug
reports, the index and the model configuration — the surface where the
vocabulary assumes you built the mesh.

Resolution is local, then owner, then anyone else declaring a model, pinned once
it answers. Pinned rather than re-derived per call: a mesh that re-decides every
request eventually decides differently mid-conversation, and a chat whose second
turn lands on another machine has no history there. Cleared only on a transport
failure, and only when there is somewhere else to go — a single-node mesh whose
model is down should say so, not report AI as unconfigured.

Plugin/unraid/api/ai.php

@@ -136,7 +136,7 @@ if ($action !== 'poll') {
 // on any host but HOST1, but a hidden link is not access control and this endpoint is reachable
 // directly. Every action is refused rather than just the expensive ones — there is no such thing
 // as a read this host is entitled to, since the index and the model are not here.
-if (!vv_is_ai_host()) {
+if (!vv_ai_is_owner()) {
     http_response_code(404);
     echo json_encode(['ok' => false, 'error' => 'AI is not available on this host']);
     exit;