fix: OnStart ctx bug, rename session→user, full logout cleanup

- Use context.Background() for cleanup goroutine instead of
  OnStart ctx which is cancelled after startup completes
- Rename GetSessionCount→GetUserCount, DeleteStaleSessions→
  DeleteStaleUsers to reflect that sessions represent users
- HandleLogout now fully cleans up when last client disconnects:
  parts all channels (notifying members via QUIT), removes
  empty channels, and deletes the session/user record
- docker build passes, all tests green, 0 lint issues

internal/db/queries.go

@@ -796,8 +796,8 @@ func (database *Database) DeleteClient(
 	return nil
 }
 
-// GetSessionCount returns the number of active sessions.
-func (database *Database) GetSessionCount(
+// GetUserCount returns the number of active users.
+func (database *Database) GetUserCount(
 	ctx context.Context,
 ) (int64, error) {
 	var count int64
@@ -808,7 +808,7 @@ func (database *Database) GetSessionCount(
 	).Scan(&count)
 	if err != nil {
 		return 0, fmt.Errorf(
-			"get session count: %w", err,
+			"get user count: %w", err,
 		)
 	}
 
@@ -838,9 +838,9 @@ func (database *Database) ClientCountForSession(
 	return count, nil
 }
 
-// DeleteStaleSessions removes clients not seen since the
-// cutoff and cleans up orphaned sessions.
-func (database *Database) DeleteStaleSessions(
+// DeleteStaleUsers removes clients not seen since the
+// cutoff and cleans up orphaned users (sessions).
+func (database *Database) DeleteStaleUsers(
 	ctx context.Context,
 	cutoff time.Time,
 ) (int64, error) {