feat: add environment variable validation on server startup

[jikrana1]

Description

Added environment variable validation that runs on server startup, before DB and Redis connection. This ensures the server fails fast with a clear error message if any required environment variable is missing or still set to a placeholder value, instead of starting silently and failing later with cryptic errors.

Changes Made

• Created validateEnv() function in config/validateEnv.js
• Required vars (MONGO_URI, UPSTASH_REDIS_REST_URL, UPSTASH_REDIS_REST_TOKEN) → if missing or placeholder, logs clear error and calls process.exit(1)
• Optional vars (PORT) → if missing, logs a warning and server continues with default
• Called validateEnv() in server.js after dotenv.config() and before any DB or Redis setup

Related Issue

Closes #54

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 34cfe1a1-c95d-4f4b-85e8-977bea1fe433

📥 Commits

Reviewing files that changed from the base of the PR and between b13fd53 and 215126c.

📒 Files selected for processing (2)

• backend/src/config/validateEnv.js
• backend/src/server.js

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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