Feat/unify env file resolution

[Opulencechuks]

Closes #255

Summary

Unifies environment file resolution and loading logic between main.ts and app.module.ts to eliminate drift in .env lookup paths.

Introduced a central helper loadEnv() in src/common/utils/env-loader.ts to coordinate search candidates deterministically. The helper enforces that .env file values take precedence over OS environment variables (dotenv always wins over the OS env).

Testing

A new test suite was created under src/common/utils/__tests__/env-loader.spec.ts asserting that:

1. Candidate search paths match the canonical priority.
2. The honest precedence rule (dotenv overrides OS env) is respected.
3. Both direct loadEnv() calls (the main.ts path) and NestJS module initialization (the app.module.ts path) agree on the final environment state.

bun test app/backend/src/common/utils/__tests__/env-loader.spec.ts

Screenshots

N/A (Backend changes only)

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.