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_test.go

@@ -25,8 +25,8 @@ func TestMigrateCreatesTablesAndVersion(t *testing.T) {
 	if err := db.QueryRow(`SELECT version FROM schema_version`).Scan(&version); err != nil {
 		t.Fatalf("query version: %v", err)
 	}
-	if version != 1 {
-		t.Errorf("version = %d, want 1", version)
+	if version != 2 {
+		t.Errorf("version = %d, want 2", version)
 	}
 
 	// Verify tables exist by inserting into them.
@@ -64,8 +64,8 @@ func TestMigrateIdempotent(t *testing.T) {
 	if err := db.QueryRow(`SELECT version FROM schema_version`).Scan(&version); err != nil {
 		t.Fatalf("query version: %v", err)
 	}
-	if version != 1 {
-		t.Errorf("version = %d after double migrate, want 1", version)
+	if version != 2 {
+		t.Errorf("version = %d after double migrate, want 2", version)
 	}
 }