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/web/static/replay.js

@@ -0,0 +1,83 @@
+// ReplayPlayer drives xterm.js playback of recorded session events.
+function ReplayPlayer(containerId, sessionId) {
+    this.terminal = new Terminal({
+        cols: 80,
+        rows: 24,
+        convertEol: true,
+        disableStdin: true,
+        theme: {
+            background: '#000000',
+            foreground: '#ffffff'
+        }
+    });
+    this.terminal.open(document.getElementById(containerId));
+
+    this.sessionId = sessionId;
+    this.events = [];
+    this.index = 0;
+    this.speed = 1;
+    this.timers = [];
+    this.playing = false;
+
+    // Fetch events immediately.
+    var self = this;
+    fetch('/api/sessions/' + sessionId + '/events')
+        .then(function(r) { return r.json(); })
+        .then(function(data) {
+            self.events = data.events || [];
+        });
+}
+
+ReplayPlayer.prototype.play = function() {
+    if (this.playing) return;
+    if (this.events.length === 0) return;
+    this.playing = true;
+    this._schedule();
+};
+
+ReplayPlayer.prototype.pause = function() {
+    this.playing = false;
+    for (var i = 0; i < this.timers.length; i++) {
+        clearTimeout(this.timers[i]);
+    }
+    this.timers = [];
+};
+
+ReplayPlayer.prototype.reset = function() {
+    this.pause();
+    this.index = 0;
+    this.terminal.reset();
+};
+
+ReplayPlayer.prototype.setSpeed = function(speed) {
+    this.speed = speed;
+    if (this.playing) {
+        this.pause();
+        this.play();
+    }
+};
+
+ReplayPlayer.prototype._schedule = function() {
+    var self = this;
+    var baseT = this.index < this.events.length ? this.events[this.index].t : 0;
+
+    for (var i = this.index; i < this.events.length; i++) {
+        (function(idx) {
+            var evt = self.events[idx];
+            var delay = (evt.t - baseT) / self.speed;
+            var timer = setTimeout(function() {
+                if (!self.playing) return;
+                // Only write output events (d=1) to terminal; input is echoed in output.
+                if (evt.d === 1) {
+                    var raw = atob(evt.data);
+                    self.terminal.write(raw);
+                }
+                self.index = idx + 1;
+                if (self.index >= self.events.length) {
+                    self.playing = false;
+                }
+            }, delay);
+            self.timers.push(timer);
+        })(i);
+    }
+};