Enable strict TypeScript mode in backend tsconfig

[Josie123-Dev]

What

Enable strictNullChecks, noImplicitAny, strictBindCallApply, forceConsistentCasingInFileNames, and noFallthroughCasesInSwitch in backend/tsconfig.json. Fix all resulting type errors across the backend codebase.

Why

The backend currently runs with nearly all strict TypeScript checks disabled (strictNullChecks: false, noImplicitAny: false). For a platform handling encrypted health credentials and vaccination records, implicit any types and unchecked nulls are a liability — they hide bugs that could lead to data corruption, unauthorized access, or silent failures in credential verification flows.

The frontend already has "strict": true. The backend should match.

Scope

In scope:

• Set all strict-family flags to true in backend/tsconfig.json
• Fix every resulting compilation error (expect ~50-150 fixes across 19 modules)
• Replace @ts-ignore in app.module.ts:62 with proper typing for the Redis cache store
• Ensure npm run build passes with zero errors after changes

Out of scope:

• Refactoring module architecture
• Adding new features
• Changing runtime behavior

Acceptance Criteria

• backend/tsconfig.json has "strict": true (or all individual strict flags enabled)
• npm run build in backend/ completes with zero TypeScript errors
• npm run test in backend/ passes all existing tests
• No @ts-ignore or @ts-nocheck comments remain
• PR includes a summary of the types of fixes applied

Technical Context

• Config file: backend/tsconfig.json
• The @ts-ignore at backend/src/app.module.ts:62 is for a Redis cache store type — use @nestjs/cache-manager types or a proper type assertion
• NestJS 10 supports strict mode well; most fixes will be null checks and explicit type annotations
• Run npx tsc --noEmit to see all errors before fixing

[Soldier224K]

Hi @Josie123-Dev, I’d like to contribute to issue #166 and take ownership of the backend TypeScript strict-mode migration.

I’ll enable the requested strict checks in backend/tsconfig.json, run npx tsc --noEmit to establish the complete error set, and systematically resolve the resulting type errors across the backend without changing runtime behavior or architecture.

My approach will include:

• Enabling strict / the specified strict compiler flags.
• Resolving nullability issues with proper guards and accurate types rather than unsafe casts.
• Replacing implicit any types with explicit, domain-appropriate types.
• Fixing strictBindCallApply, casing, and switch fallthrough issues.
• Replacing the @ts-ignore in app.module.ts with proper Redis cache-store typing.
• Removing any remaining @ts-ignore / @ts-nocheck comments.
• Running npm run build and npm run test to verify the changes.

I’ll keep the PR focused strictly on type-safety and compilation correctness, with no unrelated refactoring or feature changes.

I’d be happy to take this issue and provide a clean, reviewable PR with a summary of the fixes applied.