Fix backend startup on MongoDB connection failure

[saurabhhhcodes]

Summary

• log the actual MongoDB connection error message during startup
• terminate the backend process when the initial database connection fails
• prevent the API server from continuing in a partially initialized state without a working database

Why

If MONGO_URI is missing, invalid, or MongoDB is unavailable, continuing startup makes later API failures harder to diagnose. Failing fast gives maintainers and deployments a clear signal that the backend is not ready.

Validation

• /Users/saurabhkumarbajpaiai/.cache/codex-runtimes/codex-primary-runtime/dependencies/node/bin/node --check backend/config/db.js passed
• git diff --check passed

GSSoC labels requested

If accepted, please add gssoc:approved, level:beginner, quality:clean, and type:bug / type:backend if they match the project label scheme.

[saurabhhhcodes]

Updated this PR against the latest main. The original fail-fast behavior is now already present upstream, so I kept the PR useful by adding one small follow-up guard: fail immediately with a clear message when MONGO_URI is not configured before calling mongoose.connect.

Validation after the update:

• git diff --check origin/main...HEAD
• node --check backend/config/db.js
• backend/node_modules/.bin/eslint backend/config/db.js

All passed locally. If this remains useful for GSSoC, please consider gssoc:approved, level:beginner, quality:clean, and type:bug / backend-equivalent labels.