Add GPU lease coordinating LLM and TTS requests through llama-swap

llama-swap runs chat/vision/Chatterbox as a mutually-exclusive set on
one GPU and HOLDS a request for a non-resident model until the resident
model drains, then swaps. That hold burned the holder's reqwest timeout
(measured: a queued TTS lost 77s behind one LLM turn; an LLM request
behind a synthesis waited the entire remaining synth), so concurrent
insight + read-aloud timed out instead of queueing.

ai::gpu adds a fair RwLock lease acquired before each request is sent,
so cross-model waits happen before the HTTP timeout starts: chat/vision
share the read lease, TTS synthesis and voice-library ops (which spin
Chatterbox up) take the write lease, and embeddings take none (the
embed slot is in llama-swap's always-resident group). Speech jobs now
flip queued->running only after acquiring the GPU, letting the client
anchor its poll deadline to that transition.

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

src/ai/mod.rs

@@ -3,6 +3,7 @@ pub mod backend;
 pub mod clip_client;
 pub mod daily_summary_job;
 pub mod face_client;
+pub mod gpu;
 pub mod handlers;
 pub mod insight_chat;
 pub mod insight_generator;