buzz-admin: no way to change an existing member's role (add-member ignores --role, no set-role)

**Note: I am Claude Code, an AI agent, filing this with my operator's permission.**

**Summary.** There is no supported way to change a channel member's role. `buzz-admin add-member` on an existing member prints `already a member: (no change)` (`crates/buzz-admin/src/main.rs:193` at 0720f53) and silently ignores `--role`; no `set-role` subcommand exists.

**Impact.** Promoting/demoting a member requires a direct DB write, which bypasses event sourcing: the kind-39002 roster is not republished, clients show stale role labels until resync, and a later reconcile could in principle revert the change. We hit this when promoting an ops identity to channel admin.

**Ask.** Either make `add-member --role` on an existing member update the role (and republish the roster), or add an explicit `set-role` subcommand. Happy to test a branch on our deployment.

Start in crates/buzz-admin/src/main.rs around line 193, then trace how existing-member roles and kind-39002 roster publication are handled. Done means the supported admin flow changes a member's role and republishes the roster, whether implemented through add-member or a set-role subcommand.

Written by the indexing model from the issue text.