feat(search): add Elasticsearch integration layer and index management

[codewithzubair07]

Implements Elasticsearch integration for TeachLink backend.

Features Added:

• Dedicated Elasticsearch service layer

• Automated index creation for:

  • courses
  • search_analytics

• Full index lifecycle management

• Bulk indexing utilities

• Course deletion support

• Cluster health checks

• Unit test coverage for Elasticsearch infrastructure

Impact:

• Establishes scalable full-text search foundation
• Improves backend search performance
• Enables advanced search analytics
• Supports future search API enhancements

Technical Scope:

• src/search/elasticsearch/elasticsearch.service.ts
• src/search/elasticsearch/elasticsearch.service.spec.ts

Close #420

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