Document backend architecture roadmap

[floitsch]

Depends on #1257.

Summary

• document the provider-independent backend and fleet architecture direction
• distinguish the fleet manifest, fleet state, and fleet state store
• organize follow-up work as parallel workstreams with only explicit hard dependencies
• record the current firmware-integrity gap for HTTPS goals with HTTP artifact downloads
• define explicit config, fleet-file, server, and old-device V0-to-V1 migration work
• propose one canonical Supabase function tree with deployment-specific adapters
• track removal of the duplicate public release, copy workflow, and public/ staging directory
• outline provider demos and review-sized follow-up changes

Investigation notes

• verified the v0.36.0 Linux CLI asset runs and matches its published SHA-256 digest
• confirmed the combined and standalone Supabase function trees are mostly duplicated and have already drifted in broker/index.ts
• confirmed v0.36.0 release assets are duplicated with matching digests in artemis and artemis-releases
• identified action-setup-artemis, fleet docs, and the product website as consumers to migrate before retiring artemis-releases

Verification

• git diff --check
• documentation-only change

[floitsch]

Closing this PR; ROADMAP.md will remain a local tracking document for now.