Add player position tracking relay + rendering (Phase 7b)

Relays the mod's throttled player_positions roster over a new
/ws/players/:serverId gateway (Redis pub/sub + snapshot key so a
tab connecting between mod flushes isn't empty), gated per-server
by a new playerPositionsVisible admin toggle independent of the
mod's own tracking config. Frontend renders the roster as map
markers with a show/hide toggle and online count. Covered by unit
tests (players.test.ts, ws-gateway.test.ts, admin.test.ts) and a
new e2e spec that plays the real mod WS protocol from inside a
browser context.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015tKdPZt78zbPUZMXWzKEKt

api/src/db/schema.ts

@@ -14,6 +14,10 @@ export const servers = pgTable("servers", {
   // shared — 'journeymap' (default), 'xaero', or 'off' to disable sharing entirely. No admin UI
   // to flip this yet (Phase 6) — set directly in the DB for now, same as anonymousChatAllowed.
   waypointFormat: text("waypoint_format").notNull().default("journeymap"),
+  // Phase 7b: whether the api relays the mod's throttled player-position roster on to web
+  // viewers. Independent of the mod-local `playerTrackingEnabled` config (see MCMapperMod.java's
+  // doc comment) — this is the backend-side "should we show it" toggle, admin-configurable.
+  playerPositionsVisible: boolean("player_positions_visible").notNull().default(true),
   createdAt: timestamp("created_at", { withTimezone: true }).notNull().defaultNow(),
 });