fix(backend): add and configure ESLint for backend workspace

[Harxhit]

Summary

This PR adds and configures ESLint for the backend workspace using Flat Config (eslint.config.js). It introduces TypeScript ESLint tooling and backend-specific linting configuration without modifying shared workspace packages. Some strict TypeScript ESLint rules were temporarily relaxed to keep development unblocked while the existing codebase is gradually migrated to stricter standards.

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Type of Change

• Infrastructure / DevOps
• Refactor (no functional change)

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What Changed

• Added ESLint setup for the backend workspace.
• Configured Flat Config based ESLint setup (eslint.config.js).
• Added TypeScript ESLint plugins and supporting lint packages.
• Configured backend-only linting workflow.
• Updated root lint script to target backend workspace only.
• Temporarily disabled several strict TypeScript ESLint rules to avoid blocking development while the existing codebase is gradually migrated to stricter standards.

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How to Test

1. Install dependencies:

pnpm install

2. Run backend linting:

cd apps/backend && pnpm lint

3. Verify backend starts successfully:

pnpm --filter @devcard/backend dev

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Checklist

• My code follows the project's coding style (pnpm lint passes for backend workspace).
• TypeScript compiles without errors (pnpm -r run typecheck).
• I have added or updated tests for the changes I made.
• All tests pass locally (pnpm -r run test).
• I have updated documentation where necessary.
• No new console.log or debug statements left in the code.
• Breaking changes are documented in this PR description.

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Proof

Note -> Please ignore the neon adapter error.

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Additional Context

Some strict ESLint rules such as no-explicit-any, unsafe assignment checks, and strict boolean expression checks were temporarily turned off to avoid blocking development. These rules will be gradually re-enabled as the backend codebase is improved and properly typed.

[ShantKhatri]

@Harxhit could you please add SS of the test?

[Harxhit]

@Harxhit could you please add SS of the test?

I have added the proofs.