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/players.ts

@@ -0,0 +1,46 @@
+import { eq } from "drizzle-orm";
+import { db } from "./db/client";
+import { servers } from "./db/schema";
+import { redis } from "./redis";
+
+export interface PlayerPosition {
+  uuid: string;
+  username: string;
+  x: number;
+  y: number;
+  z: number;
+}
+
+export function playerChannel(serverId: string): string {
+  return `mcmapper:players:${serverId}`;
+}
+
+function snapshotKey(serverId: string): string {
+  return `mcmapper:players:snapshot:${serverId}`;
+}
+
+export async function arePlayerPositionsVisible(serverId: string): Promise<boolean> {
+  const [row] = await db
+    .select({ playerPositionsVisible: servers.playerPositionsVisible })
+    .from(servers)
+    .where(eq(servers.id, serverId))
+    .limit(1);
+  return row?.playerPositionsVisible ?? false;
+}
+
+/**
+ * Fans a player roster out to every browser subscribed to this server's channel, and stashes it
+ * in Redis so a browser tab that connects between mod flushes gets the current roster immediately
+ * instead of waiting up to `playerPositionIntervalTicks` for the next one — see
+ * players-gateway.ts's `open()`. No history/persistence beyond the latest snapshot: unlike chat,
+ * player positions have no meaningful "history" to keep.
+ */
+export async function publishPlayerPositions(serverId: string, dimension: number, players: PlayerPosition[]) {
+  const payload = JSON.stringify({ dimension, players });
+  await redis.set(snapshotKey(serverId), payload);
+  await redis.publish(playerChannel(serverId), payload);
+}
+
+export async function getPlayerSnapshot(serverId: string): Promise<string | null> {
+  return redis.get(snapshotKey(serverId));
+}