Research optional cloud-sync architecture

[SamDozer]

Research only. No implementation, no provider commitment.

Two rules constrain the whole design:

1. The app keeps working fully offline. Sync is optional, permanently.
2. No account is ever required to practise. An account buys sync, nothing
   else.

Questions to answer:

• The local database stays the source of truth — how does that shape the model?
• Conflict handling that cannot lose practice. Two devices, one offline for a
  week: what happens?
• What syncs — routines, history, settings, Guitar DNA. All of it? Separately
  toggleable?
• Encryption in transit and at rest
• Account deletion and full data export as first-class features, not
  afterthoughts
• Cost. This is a free app with no revenue.

Deliverable: a written proposal, not code. A new privacy policy would ship
with the feature, before a single byte leaves a device.

Part of v0.3 — see ROADMAP.md.