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>

.env.example

@@ -51,8 +51,8 @@ TYPESENSE_URL=http://localhost:8108
 BUZZ_BIND_ADDR=0.0.0.0:3000
 # Public WebSocket URL — used in NIP-42 auth challenges
 RELAY_URL=ws://localhost:3000
-# Stable relay signing key. Set this in dev if you want REST-created forum posts
-# to keep resolving to the original author across relay restarts.
+# Stable relay signing key (required). `just bootstrap` generates a random key in
+# the gitignored .env file. Preserve that value across restarts and backups.
 # BUZZ_RELAY_PRIVATE_KEY=<32-byte hex private key>
 # Optional: path to the web UI dist directory. When set, the relay serves
 # the web frontend at / for browser requests. Leave unset for local dev