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/ws-gateway.ts

@@ -4,6 +4,7 @@ import { chunkColumns, chunkSections, servers } from "./db/schema";
 import { markChunkDirty } from "./redis";
 import { storeLinkCode } from "./link";
 import { recordAndPublishChat } from "./chat";
+import { arePlayerPositionsVisible, publishPlayerPositions, type PlayerPosition } from "./players";
 
 // Wire protocol (mod <-> api), one JSON object per WS text frame:
 //
@@ -49,6 +50,14 @@ import { recordAndPublishChat } from "./chat";
 // (per-server `waypointFormat` config) and forwards the structured point — the mod owns building
 // the actual chat text in that format (see WaypointShare.java's javadoc for the researched wire
 // formats and their attribution).
+//
+//   mod -> api  {"type":"player_positions","dimension":0,"players":[{"uuid":"...","username":"...","x":..,"y":..,"z":..}]}
+//
+// Phase 7b: the mod's throttled online-player roster, always the full current list (not a diff —
+// see PlayerPosition.java's javadoc). Not persisted (no meaningful history) — just fanned out via
+// players.ts's publishPlayerPositions to whatever browsers are subscribed on /ws/players/:serverId
+// (players-gateway.ts), gated on the per-server `playerPositionsVisible` admin toggle (independent
+// of the mod-local `playerTrackingEnabled` config that decides whether this message is sent at all).
 
 interface Column {
   x: number;
@@ -202,6 +211,14 @@ export const wsGateway = {
       });
       return;
     }
+
+    if (msg.type === "player_positions") {
+      if (!(await arePlayerPositionsVisible(state.serverId))) return;
+      const dimension: number = msg.dimension;
+      const players: PlayerPosition[] = msg.players ?? [];
+      await publishPlayerPositions(state.serverId, dimension, players);
+      return;
+    }
   },
 
   close(ws: any) {