Explore Fitbit integration for smoking context

[feragusper]

Investigate whether Fitbit integration is feasible and useful for the product.

Scope:

• Evaluate technical feasibility, platform constraints, and API access requirements.
• Identify whether Fitbit data could enrich smoking insights, such as activity, sleep, or daily rhythm context.
• Define the minimum viable experience if integration is possible.
• Document limitations clearly if the integration is not realistic.

[feragusper]

Closed by tranche 4 discovery delivery in PR #156. Feasibility and MVP notes were documented for Fitbit integration.