docs: add .env.example mirroring the README environment table

[victrexfx]

Overview

This PR adds a .env.example file that mirrors the environment table in README.md, so contributors no longer have to transcribe the 27 process.env variables by hand. Each variable is grouped under the same headings as the README table, set to an obviously fake placeholder or documented default, and annotated with whether startup requires it. A short README note points at the new file to keep the two in sync.

Related Issue

Changes

📄 Example environment file

• [ADD] .env.example

  • Lists all 27 variables read through process.env.
  • Grouped with the same headings as the README environment table.
  • Each value is a placeholder or documented default — no real credentials.
  • Required variables are explicitly marked as required (including SEP10_SERVER_SECRET, JWT_SECRET, CURRENT_ENCRYPTION_KEY_VERSION, ENCRYPTION_KEY, and WHATSAPP_APP_SECRET).

• [MODIFY] README.md

  • Adds a short note in the environment section pointing contributors at .env.example so the table and file stay in sync.

• [MODIFY] src/config/env.ts

  • Aligns the variable list/annotations with the documented required-vs-optional split reflected in .env.example.

Verification Results

npm test
✅ passes locally

Acceptance Criteria	Status
.env.example lists all 27 variables read through process.env	✅ All 27 variables present
Each required variable is marked as required	✅ Required variables annotated
No value in the file is a real credential	✅ Placeholders only, safe to commit
npm test passes locally	✅ Passes

Closes #96

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