games: the felt other people can sit at, and the version that settles the race

Phase B foundation for the multiplayer casino: the shared-table storage layer,
the SSE fan-out, and the lock that only ever pretends to be the authority.

- game_tables/game_seats/game_chat, plus a nullable table_id on game_live_hands
  so occupancy stays one row per player — the same primary key that stops a
  second solo hand stops a second seat. No second uniqueness domain, no split
  brain, no cash-out-to-zero while sitting on a pot.
- The money model the plan sketched turned out simpler than it drew: chips cross
  the border only at sit-down and get-up, so a hand settles by moving the pot
  *within* the state blob and credits nobody. That deletes the payout ledger
  the design called for — there is no money write to make idempotent, only a
  state write conditional on the version. A replayed settle affects zero rows.
- CommitTable/SitDown/LeaveTable each one transaction with the state write in it;
  the version column is the concurrency authority and the striped in-memory lock
  is only an optimisation over it, because a mutex does not survive a redeploy.
- The SSE hub is a dumb byte fan-out: non-blocking sends (a stalled phone must
  not hold the table lock and freeze the clock for the room) and never a DB
  touch after the first read (holding the one connection open bricks the app).
- DueTables/PushDeadlines for the turn clock to come; Chat keeps the hand_no it
  was said during, because at a money table collusion looks like chat.

Storage and hub tested, including the version race and the never-block publish.
No handlers wired yet, so nothing a player can see has changed.

Claude-Session: https://claude.ai/code/session_013M5nD7PgUboJXoDcYHzpuJ

internal/storage/db.go

@@ -94,6 +94,9 @@ func runMigrations(d *sql.DB) error {
 	addColumnIfMissing(d, "post_log", "url_canonical", "TEXT")
 	addColumnIfMissing(d, "post_log", "forced", "INTEGER NOT NULL DEFAULT 0")
 	addColumnIfMissing(d, "round_robin_state", "last_channel", "TEXT")
+	// Occupancy of a shared table. Rows written before the casino went multiplayer
+	// are solo games and read as NULL, which is exactly what they are.
+	addColumnIfMissing(d, "game_live_hands", "table_id", "TEXT")
 
 	// FTS5 virtual tables don't support IF NOT EXISTS reliably.
 	// Check sqlite_master before creating.