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/e2e-git-perms.sh

@@ -336,6 +336,7 @@ export BUZZ_GIT_HOOK_HMAC_SECRET="${HMAC_SECRET}"
 export BUZZ_BIND_ADDR="${RELAY_HOST}:${RELAY_PORT}"
 export RELAY_URL="${RELAY_WS}"
 export RUST_LOG="buzz_relay=warn"
+export BUZZ_RELAY_PRIVATE_KEY="${BUZZ_RELAY_PRIVATE_KEY:-$(openssl rand -hex 32)}"
 export BUZZ_REQUIRE_AUTH_TOKEN=false
 
 # Clean repos dir (isolated test state)