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/web/server.go

@@ -76,6 +76,13 @@ type Server struct {
 	metricsMu sync.Mutex
 	saltDay   int64
 	salt      [16]byte
+
+	// The shared-table machinery. hub fans SSE frames out to the phones at a felt;
+	// tableLocks is the striped optimisation over the DB's version column (see
+	// games_table.go). Both are nil-safe to construct always: they cost nothing
+	// until a table is opened.
+	hub        *gamesHub
+	tableLocks *stripedLocks
 }
 
 // New builds the server. Templates are parsed once at startup. Each page gets
@@ -136,7 +143,7 @@ func New(cfg config.WebConfig, sources []config.SourceConfig, postingEnabled boo
 		live = append(live, ch)
 	}
 
-	s := &Server{cfg: cfg, sources: infos, postingEnabled: postingEnabled, tpls: tpls, adminSubs: adminSubs, adv: adv, advPost: advPost, channels: live}
+	s := &Server{cfg: cfg, sources: infos, postingEnabled: postingEnabled, tpls: tpls, adminSubs: adminSubs, adv: adv, advPost: advPost, channels: live, hub: newGamesHub(), tableLocks: newStripedLocks()}
 
 	// Optional OIDC sign-in (Authentik). Discovery is a network call; if the
 	// provider is unreachable at boot we log and serve anonymously rather than