chore(backend): formalize API v1 versioning and deprecation headers (#39)

[Adedayo-Data]

Description

Refactored the API architecture to use a versioned mounting strategy under /api/v1. This PR also introduces standard headers to support future deprecation and sunset policies without breaking changes.

Key Changes

• Master V1 Router: Created src/api/v1/router.ts to centralize all v1 logic.
• Versioned Mounting: Migrated stream management routes from the root to /api/v1/streams.
• Infrastructure Headers: Added a global middleware to the v1 router that injects:
  • X-API-Version: v1
  • Deprecation: false (Prepared for future lifecycle management).
• Global Routes: Kept /health and /webhooks at the root level for infrastructure/webhook compatibility.

Verification

• curl -I: Confirmed versioning headers are present in responses.
• npm test: Verified all stream tests now point to the versioned paths.

closes #39

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