Remove public relay signing key fallback (#6729)

Require `BUZZ_RELAY_PRIVATE_KEY` on every relay startup and remove the
shared public fallback key completely.

For local development, `just bootstrap` now generates a random key once
in the gitignored `.env` file. Re-running bootstrap preserves that key,
so relay-authored events retain the same identity across restarts. The
local relay recipes reload `.env`, while isolated CI and E2E launchers
pass explicit per-run keys.

Startup fails immediately when the key is missing or invalid, before
connecting to Postgres or Redis. Deployed relays continue to receive
their stable key from the existing chart-managed secret.

Validated with:

- `./scripts/test-ensure-local-relay-key.sh`
- `cargo test -p buzz-relay --bin buzz-relay`
- `cargo clippy -p buzz-relay --bin buzz-relay -- -D warnings`
- `cargo fmt --all -- --check`
- `shellcheck scripts/ensure-local-relay-key.sh
scripts/test-ensure-local-relay-key.sh`

Signed-off-by: Jordan Mecom <jm@squareup.com>

scripts/start-isolated-test-relay.sh

@@ -133,6 +133,7 @@ ok "Relay built"
 # survives (same pattern the perf stack uses). Logs to ${RELAY_LOG}.
 RELAY_LOG="${RELAY_LOG:-/tmp/dawn-relay-run.log}"
 TMUX_SESSION="${TMUX_SESSION:-dawn-relay}"
+RELAY_PRIVATE_KEY="$(openssl rand -hex 32)"
 tmux kill-session -t "${TMUX_SESSION}" 2>/dev/null || true
 if command -v lsof >/dev/null 2>&1 && lsof -nP -iTCP:"${RELAY_MAIN}" -sTCP:LISTEN >/dev/null 2>&1; then
   err "Port ${RELAY_MAIN} is already in use; refusing to report a stale relay as this harness."
@@ -151,6 +152,7 @@ tmux new-session -d -s "${TMUX_SESSION}" "cd '${REPO_ROOT}' && env \
   BUZZ_S3_ACCESS_KEY=buzz_dev \
   BUZZ_S3_SECRET_KEY=buzz_dev_secret \
   BUZZ_S3_BUCKET=buzz-media \
+  BUZZ_RELAY_PRIVATE_KEY=${RELAY_PRIVATE_KEY} \
   BUZZ_REQUIRE_AUTH_TOKEN=false \
   BUZZ_RECONCILE_CHANNELS=true \
   './target/${CARGO_TARGET_PROFILE}/buzz-relay' > '${RELAY_LOG}' 2>&1"