fix(relay): publish owner roles on open relays

[tlongwell-block]

Summary

• publish the relay-signed membership role snapshot on startup when a stable relay key exists, even when NIP-43 access enforcement is disabled
• keep role metadata current after community provisioning and ownership transfer on open relays
• let existing clients discover owner/admin capabilities and show owner-only settings such as the workspace icon editor

This does not enable membership enforcement or advertise NIP-43 support on open relays; it only publishes the existing signed role metadata used for capability discovery.

Validation

• bin/cargo fmt --check
• bin/cargo check -p buzz-relay
• targeted config test passes
• relay lib suite: 654 passed; one unrelated mesh-demo timeout failed twice (demo_join_forwarded_arm_round_trips_echo, expected 200 got 504)
• pre-commit hooks passed (Rust/Desktop Tauri formatting, web/desktop Biome + file-size checks, Flutter analyze)
• pre-push client suites passed (desktop 1398, mobile, desktop-tauri 1398 + diagnostics); aggregate Rust runner was blocked first by unactivated Hermit rustc 1.89, then by a concurrent native-build output-dir race after retry with Hermit. Relevant relay compile/tests above are green.

[wesbillman]

This fixes the missing admin-controls path for open relays with configured stable relay and owner keys. The desktop gates relay administration on the current identity appearing as owner/admin in kind 13534; open relays currently seed that DB role but do not publish the snapshot. Publishing it at startup and after provisioning/ownership transfer restores capability discovery without enabling admission enforcement or advertising NIP-43. The config predicate is the right distinction because relay-signed metadata requires a stable key. Approving; merge after CI is green.