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/process.go

@@ -414,6 +414,9 @@ func (p *Process) stopCommand() {
 	stopStartTime := time.Now()
 	defer func() {
 		p.proxyLogger.Debugf("<%s> stopCommand took %v", p.ID, time.Since(stopStartTime))
+
+		// free the buffer in processLogger so the memory can be recovered
+		p.processLogger.Clear()
 	}()
 
 	p.cmdMutex.RLock()
@@ -646,6 +649,11 @@ func (p *Process) cmdStopUpstreamProcess() error {
 	return nil
 }
 
+// Logger returns the logger for this process.
+func (p *Process) Logger() *LogMonitor {
+	return p.processLogger
+}
+
 var loadingRemarks = []string{
 	"Still faster than your last standup meeting...",
 	"Reticulating splines...",
@@ -864,7 +872,6 @@ func (s *statusResponseWriter) WriteHeader(statusCode int) {
 	s.Flush()
 }
 
-// Add Flush method
 func (s *statusResponseWriter) Flush() {
 	if flusher, ok := s.writer.(http.Flusher); ok {
 		flusher.Flush()