docs: add environment variables documentation and example file for backend setup

[firoziya]

Related Issue

• Closes: Add Environment Variables Documentation to README #675

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Description

This PR addresses the missing documentation for backend environment variables, which currently causes setup friction for new contributors.

Changes Made:

1. Updated README.md: Added a dedicated "🔐 Environment Variables" section under the Setup Guide, including a table explaining each variable (MONGO_URI, SESSION_SECRET, PORT, LOG_LEVEL).
2. Created backend/.env.example: Provides a template file with placeholder values that contributors can copy to .env.

These changes align with the issue's suggestions and will significantly improve the local development setup experience.

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How Has This Been Tested?

• Documentation: Verified that the markdown formatting renders correctly in a local preview.
• .env.example: Confirmed that all variable names match those used in backend/server.js and backend/config/passportConfig.js.
• Process: Manually simulated a contributor's setup by copying .env.example to .env, populating dummy values, and successfully starting the backend server.

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Screenshots (if applicable)

New README Section Preview:

New .env.example file:

# Backend Environment Variables

# MongoDB Connection URI (required)
MONGO_URI=mongodb://127.0.0.1:27017/github_tracker

# Session Secret for express-session (required - use a strong random string)
SESSION_SECRET=your_super_secret_key_change_this_in_production

# Server Port (optional, defaults to 5000)
PORT=5000

# Log Level (optional: debug, info, warn, error)
LOG_LEVEL=debug

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->

## Summary by CodeRabbit

* **Documentation**
  * Added a comprehensive environment variables configuration guide with step-by-step setup instructions for creating the required backend configuration file.
  * Included a pre-configured environment variables template containing all necessary settings for database connectivity and session management, along with optional configurations for server port and logging level.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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[coderabbitai]

📝 Walkthrough

Walkthrough

This PR adds documentation and a template for backend environment variables to improve contributor onboarding. A new README section lists required and optional configuration variables alongside their defaults, and a corresponding backend/.env.example file provides a ready-to-use template with comments and example values.

Changes

Environment Variables Setup

Layer / File(s)	Summary
Environment Variables Documentation and Template backend/.env.example, README.md	README introduces an "Environment Variables" section documenting MONGO_URI, SESSION_SECRET, PORT, and LOG_LEVEL with descriptions and defaults. .env.example provides a commented template with example values for each variable, instructing users to copy from this template.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

🐰 Hop along, dear contributor mine,
Variables all in a row, so fine!
Copy from .env, a template divine,
MongoDB, secrets, and logs align! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main change: adding environment variables documentation and an example file for backend setup.
Description check	✅ Passed	The description comprehensively covers all template sections with detailed changes, testing methodology, screenshots, and type of change marked as documentation update.
Linked Issues check	✅ Passed	All requirements from issue #675 are met: Environment Variables section added to README with variable explanations, backend/.env.example file created with required variables and values.
Out of Scope Changes check	✅ Passed	All changes are directly related to issue #675 objectives: README documentation and .env.example file for environment variable setup.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

🧹 Nitpick comments (1)

backend/.env.example (1)

14-14: 💤 Low value

Add trailing newline at end of file.

The file should end with a newline character per POSIX convention. This prevents diff noise and potential linter warnings.

📝 Proposed fix

 # Log Level (optional: debug, info, warn, error)
 LOG_LEVEL=debug
+

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@backend/.env.example` at line 14, The file backend/.env.example is missing a
trailing newline; update the file so it ends with a single newline character
(ensure your editor saves a newline at EOF or add one in your commit) to satisfy
POSIX conventions and remove diff/linter noise.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@backend/.env.example`:
- Line 14: The file backend/.env.example is missing a trailing newline; update
the file so it ends with a single newline character (ensure your editor saves a
newline at EOF or add one in your commit) to satisfy POSIX conventions and
remove diff/linter noise.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: f65f2cd9-36c0-45b5-8df5-3081ad507967

📥 Commits

Reviewing files that changed from the base of the PR and between 53f820b and fc67e0e.

📒 Files selected for processing (2)

• README.md
• backend/.env.example