feat(relay): add atomic create-only community provisioning

[kalvinnchau]

Summary

• add an explicit create_only operator provisioning mode
• atomically insert the community and initial owner in one transaction
• return 409 Conflict when the host already exists instead of rotating ownership
• preserve the existing convergence mode for deployment-root tooling

Why

Kgoose provisions communities for end users. The existing ensure-and-rotate behavior allows an availability/create race to rotate an existing community owner. This closes that race at the database boundary.

Testing

• cargo fmt --all
• cargo check -p buzz-db -p buzz-relay
• cargo test -p buzz-db create_community_with_owner_is_atomic_and_create_only -- --ignored --nocapture
• cargo test -p buzz-relay operator --no-fail-fast
• pre-push Rust and desktop-Tauri suites passed; JS/mobile hooks could not run because pnpm, dart, and flutter are unavailable locally

[baxen]

LGTM

[baxen]

Approved after end-to-end review with squareup/cash-server#104238. Atomic create-only provisioning, natural same-owner retry behavior, management-origin NIP-98 binding, and conflict semantics look correct.