Update sharing spec with strategic alignment analysis

Commit notes

- Add section on how sharing fits the all-in-one vision - Map sharing features to producer jobs-to-be-done - Add feature gating by subscription tier (Free/Pro/Label) - Include concrete use cases for music producers - Add competitive positioning analysis - Include risk assessment and final recommendation - Clarify upgrade prompts and revenue alignment

Co-authored-by: armin.naimi <[email redacted]>