fix: reconstruct corrupted server.js message pipeline

[ayomidemariam]

Summary

Reconstructs the fatally corrupted src/server.js which had duplicate wss declarations causing SyntaxError: Identifier 'wss' has already been declared, preventing 6 test suites from loading.

Root Cause

The upstream server.js contained two interleaved implementations merged incorrectly:

• First const wss = new WebSocketServer(...) on line 31 bound to server
• Second const wss = new WebSocketServer({ noServer: true }) on line 53
• Two HTTP server instances (server and httpServer)
• clearInterval(interval) referencing undefined variable (should be heartbeatInterval)
• Missing createRateLimiter import and safeSend function

Changes

• Removed duplicate declarations: Single const wss bound to httpServer
• Merged HTTP endpoints: Supports /health (returns { status: "OK" }) and /healthz (returns { status: "ok", uptime }) for backward compatibility
• Added /readyz readiness endpoint with connection/room counts
• Added /metrics Prometheus-format endpoint tracking connections, messages, auth failures, heap usage
• Added markShuttingDown() method for graceful shutdown (503 responses)
• Restored safeSend wrapper for error-safe WebSocket sends
• Restored per-message rate limiting via createRateLimiter import
• Fixed clearInterval referencing correct heartbeatInterval variable
• Added per-IP connection tracking with configurable maxConnectionsPerIp
• Returns { wss, httpServer, rooms, markShuttingDown } supporting both wss.address() and httpServer.address() patterns

Verification

• All 20 test files pass (178 tests passed, 15 skipped for postgres)
• Lint passes with zero errors
• No existing test files modified

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Closes #191