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/storage/migrations/002_session_events.sql

@@ -0,0 +1,9 @@
+CREATE TABLE session_events (
+    id         INTEGER PRIMARY KEY AUTOINCREMENT,
+    session_id TEXT    NOT NULL REFERENCES sessions(id) ON DELETE CASCADE,
+    timestamp  TEXT    NOT NULL,
+    direction  INTEGER NOT NULL,
+    data       BLOB    NOT NULL
+);
+
+CREATE INDEX idx_session_events_session_id ON session_events(session_id);