Write a SpacetimeDB backend module in C# that maintains a table of currently connected players using lifecycle reducers.

TABLE
- OnlinePlayer (public)
  - Fields:
    - Identity: Identity (primary key)
    - ConnectedAt: Timestamp

REDUCERS
- ClientConnected (ReducerKind.ClientConnected): inserts an OnlinePlayer row using ctx.Sender as the identity and ctx.Timestamp as ConnectedAt
- ClientDisconnected (ReducerKind.ClientDisconnected): deletes the OnlinePlayer row for ctx.Sender
