proxy: Improve logging performance and allow separate log streaming (#421)

Replace container/ring.Ring with a custom circularBuffer that uses a
single contiguous []byte slice. This fixes the original implementation
which created 10,240 ring elements instead of 10KB of storage.

GetHistory is now 139x faster (145μs → 1μs) and uses 117x less memory
(1.2MB → 10KB). Allocations reduced from 2 to 1 per write operation.

Create a LogMonitor per proxy.Process, replacing the usage
of a shared one. The buffer in LogMonitor is lazy allocated on the first
call to Write and freed when the Process is stopped. This reduces
unnecessary memory usage when a model is not active.

The /logs/stream/{model_id} endpoint was added to stream logs from a
specific process.

proxy/processgroup.go

@@ -46,7 +46,8 @@ func NewProcessGroup(id string, config config.Config, proxyLogger *LogMonitor, u
 	// Create a Process for each member in the group
 	for _, modelID := range groupConfig.Members {
 		modelConfig, modelID, _ := pg.config.FindConfig(modelID)
-		process := NewProcess(modelID, pg.config.HealthCheckTimeout, modelConfig, pg.upstreamLogger, pg.proxyLogger)
+		processLogger := NewLogMonitorWriter(upstreamLogger)
+		process := NewProcess(modelID, pg.config.HealthCheckTimeout, modelConfig, processLogger, pg.proxyLogger)
 		pg.processes[modelID] = process
 	}
 
@@ -88,6 +89,13 @@ func (pg *ProcessGroup) HasMember(modelName string) bool {
 	return slices.Contains(pg.config.Groups[pg.id].Members, modelName)
 }
 
+func (pg *ProcessGroup) GetMember(modelName string) (*Process, bool) {
+	if pg.HasMember(modelName) {
+		return pg.processes[modelName], true
+	}
+	return nil, false
+}
+
 func (pg *ProcessGroup) StopProcess(modelID string, strategy StopStrategy) error {
 	pg.Lock()