#644 Refactor research audience handling and validation

• Updated getResearchAudienceHandler to use the new validateGetResearchAudienceRequest for request validation.
• Introduced researchAudiencePlatforms.ts to define supported audience platforms.
• Added validateGetResearchAudienceRequest to validate incoming requests for the audience endpoint, ensuring proper platform handling and defaults.
• Created tests for validateGetResearchAudienceRequest to cover various scenarios including platform validation and defaults.
• Refactored mapArtistAudienceSource to utilize the new platform definitions.

These changes enhance the clarity and maintainability of the audience research functionality.

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Summary by cubic

Centralized audience platform handling and validation. Defaults platform to instagram, rejects invalid values, and hardens mapping against prototype keys.

• Refactors
  • Added researchAudiencePlatforms.ts mapping allowed platforms to SongStats sources.
  • Introduced validateGetResearchAudienceRequest to validate auth/artist and platform; defaults to instagram and returns 400 on invalid input.
  • Updated getResearchAudienceHandler to use the new validator and validated.platform in the SongStats path.
  • Updated mapArtistAudienceSource to use the shared enum and Object.hasOwn to avoid prototype keys; added tests for known/unknown platforms and prototype handling.

Summary by CodeRabbit

• Refactor
  • Centralized platform validation for research audience queries with enhanced error messaging for unsupported platforms.