fix: resolve issues #260, #261, #274, #276

[TomikeDS]

Linked Issue

Closes #260
Closes #261
Closes #274
Closes #276

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What does this PR do?

• Empty CreateAssessmentDto Validation #260: Verified CreateAssessmentDto and UpdateAssessmentDto already have full validation decorators (IsString, IsNotEmpty, IsOptional, IsEnum, IsUUID, IsNumber, Min, Max). No changes required.

• Error Handling Consistency #261: Add custom exception classes in src/common/exceptions/app.exceptions.ts (ResourceNotFoundException, ForbiddenOperationException, ResourceConflictException, BusinessValidationException, ServiceUnavailableException) for consistent error handling across services.

• Database Connection Pool Configuration #274: Add DB connection pool Prometheus metrics (db_pool_connections_acquired_total, db_pool_connections_released_total, db_pool_size) to MetricsCollectionService. Wire pool event hooks in app.module.ts and log pool config at startup.

• Caching Strategy Documentation #276: Document caching strategy (cache-aside pattern, TTL policy table, key versioning, event-driven invalidation) in caching.constants.ts.

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Type of change

• ✨ New feature (non-breaking change that adds functionality)
• 🐛 Bug fix (non-breaking change that fixes an issue)
• 💥 Breaking change (fix or feature that changes existing API behaviour)
• ♻️ Refactor (no functional change, no new feature)
• 🧪 Tests only (no production code changes)
• 📝 Documentation only
• 🔧 Chore (build, dependencies, CI config)

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Pre-merge checklist (required)

Do not remove items. Unchecked items without an explanation will block merge.

Branch & metadata

• Branch name follows feature/issue-<N>-<slug> / fix/issue-<N>-<slug> convention
• Branch is up to date with the target branch (develop or main)
• All commits and the PR title follow the Conventional Commits format with issue reference

Code quality & tests

• npm run lint:ci — zero ESLint warnings
• npm run format:check — Prettier reports no changes needed
• npm run typecheck — zero TypeScript errors
• npm run test:ci — all tests pass, coverage ≥ 70%
• New service methods have corresponding .spec.ts unit tests
• New API endpoints are covered by at least one e2e test
• No existing tests were deleted (if any were, justification is provided in the PR description)

Error handling & NestJS best practices

• All new/updated DTOs use class-validator / class-transformer decorators and are wired through NestJS pipes (e.g. global ValidationPipe or explicit)
• All controller entry points validate external input at the boundary (no unvalidated raw any/unknown reaching the domain)
• Controllers/services throw appropriate NestJS HTTP exceptions (e.g. BadRequestException, UnauthorizedException, ForbiddenException, NotFoundException) instead of generic Error
• Any new error shapes are handled by existing exception filters or the filters have been updated accordingly
• Logging goes through the shared logging abstraction (e.g. Nest Logger or central logger service) with meaningful, structured messages
• Authentication/authorization guards (e.g. AuthGuard, role/permissions guards, custom guards) are applied to all new/modified endpoints where appropriate
• If an endpoint is intentionally public, this is explicitly mentioned in the PR description with rationale

API documentation / Swagger

• Swagger / OpenAPI decorators are added or updated for all new/changed controller endpoints (including DTOs, responses, and error schemas)
• I have started the app locally and confirmed the /api (or Swagger UI) reflects new/changed endpoints correctly
• If there are no API surface changes, this is explicitly stated in the PR description

Breaking changes

• This PR does not introduce a breaking API change
• OR: this PR introduces a breaking change and it is documented below, with migration notes

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Breaking change description (if applicable)

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Test evidence (required)

Commands run locally

# Example (edit as needed)
npm run lint:ci
npm run format:check
npm run typecheck
npm run test:ci

Manual / API verification

# Example: describe manual tests, curl commands, or Postman collections used

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Screenshots / recordings (if applicable)

[drips-wave]

@TomikeDS Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

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[RUKAYAT-CODER]

Kindly resolve conflict