feat: add session replay with terminal playback via xterm.js

Persist byte-level I/O events from SSH sessions to SQLite and add a web
UI to replay them with original timing. Events are buffered in memory
and flushed every 2s to avoid blocking SSH I/O on database writes.

- Add session_events table (migration 002)
- Add SessionEvent type and storage methods (SQLite + MemoryStore)
- Change RecordingChannel to support multiple callbacks
- Add EventRecorder for buffered event persistence
- Add session detail page with xterm.js terminal replay
- Add /api/sessions/{id}/events JSON endpoint
- Linkify session IDs in dashboard and active sessions
- Vendor xterm.js v5.3.0

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

internal/shell/recorder.go

@@ -9,12 +9,11 @@ import (
 // direction is 0 for input (client→server) and 1 for output (server→client).
 type EventCallback func(ts time.Time, direction int, data []byte)
 
-// RecordingChannel wraps an io.ReadWriteCloser and optionally invokes a callback
-// on every Read (input) and Write (output). Phase 2.3 will add byte-level
-// keystroke recording here without changing any shell code.
+// RecordingChannel wraps an io.ReadWriteCloser and optionally invokes callbacks
+// on every Read (input) and Write (output).
 type RecordingChannel struct {
-	inner    io.ReadWriteCloser
-	callback EventCallback
+	inner     io.ReadWriteCloser
+	callbacks []EventCallback
 }
 
 // NewRecordingChannel returns a RecordingChannel wrapping rw.
@@ -22,28 +21,40 @@ func NewRecordingChannel(rw io.ReadWriteCloser) *RecordingChannel {
 	return &RecordingChannel{inner: rw}
 }
 
-// WithCallback sets the event callback and returns the RecordingChannel for chaining.
+// WithCallback clears existing callbacks, sets the given one, and returns the
+// RecordingChannel for chaining. Kept for backward compatibility.
 func (r *RecordingChannel) WithCallback(cb EventCallback) *RecordingChannel {
-	r.callback = cb
+	r.callbacks = []EventCallback{cb}
 	return r
 }
 
+// AddCallback appends an additional event callback.
+func (r *RecordingChannel) AddCallback(cb EventCallback) {
+	r.callbacks = append(r.callbacks, cb)
+}
+
 func (r *RecordingChannel) Read(p []byte) (int, error) {
 	n, err := r.inner.Read(p)
-	if n > 0 && r.callback != nil {
+	if n > 0 && len(r.callbacks) > 0 {
+		ts := time.Now()
 		cp := make([]byte, n)
 		copy(cp, p[:n])
-		r.callback(time.Now(), 0, cp)
+		for _, cb := range r.callbacks {
+			cb(ts, 0, cp)
+		}
 	}
 	return n, err
 }
 
 func (r *RecordingChannel) Write(p []byte) (int, error) {
 	n, err := r.inner.Write(p)
-	if n > 0 && r.callback != nil {
+	if n > 0 && len(r.callbacks) > 0 {
+		ts := time.Now()
 		cp := make([]byte, n)
 		copy(cp, p[:n])
-		r.callback(time.Now(), 1, cp)
+		for _, cb := range r.callbacks {
+			cb(ts, 1, cp)
+		}
 	}
 	return n, err
 }