internal/router: priority queues so batch jobs yield to interactive requests

The GPU is a size-1 resource, so a single long job monopolises the box for its
whole duration and every interactive request queues behind it. Callers can now
declare intent with an X-LlamaSwap-Priority header and the serial scheduler
dispatches by score instead of by arrival.

- X-LlamaSwap-Priority: signed integer, 0 default, absent/unparseable means 0.
  interactive/normal/batch aliases resolve to +100/0/-100. Values are not
  clamped: the caller composes band and any per-user offset itself.
- serial dispatch score = priority + swap affinity + aging. Bands sit 100 apart
  so a small caller offset orders work inside a band without crossing one;
  aging is unbounded so low-priority work cannot starve.
- routing.scheduler.settings.serial.{agingDivisor,swapAffinityBonus}, defaulting
  to 60s/point and +10. swapAffinityBonus is capped at 99 so it can never
  promote a request into the next band.
- fifo adds the header to its per-model priority, so the header is not silently
  ignored under that scheduler.
- /metrics exports per-band queue depth, oldest wait and dispatch counts, plus
  counters for how often aging or swap affinity changed the pick. Each request
  records its priority, band, score and queue wait in the activity log.

Note swapAffinityBonus defaults to 10, so equal-priority requests for the
already-loaded model now run before older requests that need a swap. Set it to
0 for the previous strict arrival order.

fixes #9

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01WUyhZBgv8BBCC5MduX88gE

internal/router/scheduler/fifo.go

@@ -280,7 +280,7 @@ func (s *FIFO) grantHandler(req HandlerReq, modelID string) {
 		return
 	}
 
-	if err := shared.SetReqData(req.Ctx, "fifo_priority", strconv.Itoa(s.cfg.Priority[req.Model])); err != nil {
+	if err := shared.SetReqData(req.Ctx, "fifo_priority", strconv.Itoa(s.priorityOf(req))); err != nil {
 		s.logger.Debugf("failed to set fifo_priority metadata: %v", err)
 	}
 
@@ -311,15 +311,22 @@ func (s *FIFO) startSwap(initial HandlerReq, evict, running []string) {
 	s.effects.StartSwap(initial.Model, evict)
 }
 
+// priorityOf is a request's effective queue priority: the caller's
+// X-LlamaSwap-Priority plus the model's configured priority. Both default to 0,
+// so a deployment that uses neither keeps plain FIFO order.
+func (s *FIFO) priorityOf(req HandlerReq) int {
+	return req.Priority + s.cfg.Priority[req.Model]
+}
+
 // enqueue inserts req into the queue in priority order: it goes just before the
-// first queued item whose priority is strictly lower, so higher-priority models
-// are serviced first while equal-priority requests keep their arrival (FIFO)
-// order. Priorities come from the FifoConfig; unlisted models default to 0.
+// first queued item whose priority is strictly lower, so higher-priority
+// requests are serviced first while equal-priority requests keep their arrival
+// (FIFO) order.
 func (s *FIFO) enqueue(req HandlerReq) {
-	p := s.cfg.Priority[req.Model]
+	p := s.priorityOf(req)
 	i := len(s.queued)
 	for j, q := range s.queued {
-		if s.cfg.Priority[q.Model] < p {
+		if s.priorityOf(q) < p {
 			i = j
 			break
 		}