Feature/unify env loader

[Devadakene]

Closes #255
Summary

This PR consolidates and unifies the environment file resolution logic. Previously, .env file resolution was duplicated in both main.ts and app.module.ts with minor differences in search order, introducing potential configuration drift between application bootstrap and Nest module initialization.

A new loadEnv() helper has been introduced in src/common/utils/env-loader.ts to unify the candidate search order and loading. Both entry points now call this helper.

OS environment variables take precedence over dotenv variables (default dotenv behavior), which has been documented.

Testing

Tested using unit tests in test/env-loader.spec.ts to assert that both call paths agree on the resolved path and load variables consistently.

bun test test/env-loader.spec.ts

Output:

bun test v1.3.14 (0d9b296a)

test\env-loader.spec.ts:
(pass) loadEnv and Call Path Agreement > should resolve process.cwd() / .env if it exists [0.79ms]
(pass) loadEnv and Call Path Agreement > should fallback to app/backend/.env if process.cwd()/.env does not exist [0.25ms]
(pass) loadEnv and Call Path Agreement > should default to first candidate if none exist [0.20ms]
(pass) loadEnv and Call Path Agreement > asserts both call paths (main.ts loading and app.module.ts config path) agree on the final env state [0.32ms]

 4 pass
 0 fail
 8 expect() calls
Ran 4 tests across 1 file. [171.00ms]

Screenshots

Checklist

• Tests added or updated for new/changed behavior
• Existing tests pass
• No secrets, keys, or seed phrases committed
• Follows the coding conventions in the relevant service README
• PR is focused on a single concern

[kilodesodiq-arch]

Awesome work on this! Thanks for the contribution 🚀 Merging now.

[kilodesodiq-arch]

Hi 👋 Thanks for this PR! There are some merge conflicts with the current main. Could you rebase onto main and push when you have a moment? Once it's clean we'll get it merged 🚀