feat: Implement Core Features (Pagination, Standardized Responses, Security Headers, Database Indexing)

[SharifIbrahimDev]

Description

This PR resolves four major feature and optimization requests for the SkillSync_Server backend application:

1. Pagination Utility Service (Resolves Pagination utility service #724)

   • Introduced a reusable PaginationService accompanied by PaginationQueryDto and PaginatedResponse interface.
   • Configured robust edge-case handling (e.g., negative limits, max limits), offset/limit bounds, and automatic pagination link generation.

2. Common API Response Formatter (Resolves Common API response formatter #725)

   • Created a global ResponseInterceptor to encapsulate successful API responses into a standard, uniform envelope (success: true).
   • Modified the HttpExceptionFilter to emit a symmetrical failure format (success: false) and inject tracking elements like requestId.

3. Database Indexing Optimization (Resolves Database indexing optimization #726)

   • Appended significant structural indexes (such as isVerified, createdAt, walletAddress on User and mentorId, dayOfWeek on AvailabilitySlot).
   • Generated a formal TypeORM migration file representing the optimized database structure to improve query and lookup times.

4. Security Headers with Helmet (Resolves Security headers with Helmet #733)

   • Globally integrated Helmet middleware.
   • Enforced rigorous CSP constraints, enabled X-Frame-Options: DENY, set HSTS directives for production scaling, and eradicated the vulnerable X-Powered-By default header.

Validation Performed

• Validated new utilities via jest with explicit tests written for pagination.service and response.interceptor.
• Fixed underlying broken imports and string formats within legacy authentication test suites to guarantee a 100% CI pass rate (13/13 passing test suites).
• Built and staged database schema transformations accurately using TypeORM CLI.

Linked Issues

Closes #724
Closes #725
Closes #726
Closes #733

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[nafiuishaaq]

@SharifIbrahimDev please resolve conflict

[SharifIbrahimDev]

conflict resolved