Move request queuing from the engine to the front so the engine does only
generation. New codai/frontproxy/reqqueue.py FrontQueue: a per-model concurrency
gate sized to the model's max_instances (per-model from models.json, else the
global models.max_model_instances). A text-generation request acquires a slot
before dispatch; when all slots are busy it waits (FIFO) and shows as "queued"
with its position on the Tasks page; beyond server.queue_max_size waiting it gets
HTTP 503. A client disconnect while queued cancels the wait and drops it from the
queue with no slot leak.

Wired into both dispatch paths in app.py (direct proxy + broker route) so direct
and brokered requests share one queue, scoped to text generation only (images/
audio/embeddings pass through unqueued). Slots are released on stream completion,
send failure, or disconnect. Queue key is the canonical model id so all aliases/
paths of a model share one gate. Front never over-subscribes the engine, so the
engine's own queue stays a safety net that effectively never fills.

The engine-side QueueManager + KV-affinity instance pool are intentionally left
in place for now (trimming them is a follow-up once this is verified live).

Verified: FrontQueue unit tests (capacity, FIFO hand-off, QueueFull, cancel
cleanup, position, capacity>1) and a FrontProxy integration test (capacity
resolution, alias-key collapsing, queued task surfacing).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011DDv7BchtZQWsnPG6Jm49m