feat(poller): add SSH relay server with WebSocket-to-PTY bridge

Implements the SSH relay server (Task 2.1) that validates single-use
Redis tokens via GETDEL, dials SSH to the target device with PTY,
and bridges WebSocket binary/text frames to SSH stdin/stdout/stderr
with idle timeout and per-user/per-device session limits.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

poller/internal/sshrelay/session.go

@@ -0,0 +1,28 @@
+package sshrelay
+
+import (
+	"context"
+	"sync/atomic"
+	"time"
+
+	"golang.org/x/crypto/ssh"
+)
+
+type Session struct {
+	ID         string
+	DeviceID   string
+	TenantID   string
+	UserID     string
+	SourceIP   string
+	StartTime  time.Time
+	LastActive int64 // atomic, unix nanoseconds
+	sshClient  *ssh.Client
+	sshSession *ssh.Session
+	ptyCols    int
+	ptyRows    int
+	cancel     context.CancelFunc
+}
+
+func (s *Session) IdleDuration() time.Duration {
+	return time.Since(time.Unix(0, atomic.LoadInt64(&s.LastActive)))
+}