dev mode: say SigV4 authentication in the remaining dev-mode texts #931

#897 changes the dev-mode comments in crates/bin/Cargo.toml and crates/app/Cargo.toml to say "SigV4 authentication still enforced". A review comment on #897 asked for "SigV4 authentication" instead of "SigV4" alone. These texts on main still use the short form:

• Dockerfile.dev:10: "open authorization (SigV4 is still enforced)"
• crates/app/src/cmd_serve.rs:404, in the dev-mode startup banner: "(SigV4 still enforced)"
• crates/server/src/serve.rs:109, a doc comment: "SigV4 still verified"
• crates/server/src/serve.rs:431, the dev-mode warning in the log: "(SigV4 still enforced)"

docker-compose.dev.yml:11-12 says "SigV4 signatures are still checked for shape". Dev mode verifies the signature of each request against the seeded credentials. crates/server/src/serve.rs:424 says so: "SigV4 verification is unchanged; only the IAM policy decision is opened." So the compose comment understates the check.

Suggested change: say "SigV4 authentication" in each of these texts. In the compose file, say that dev mode still verifies the SigV4 signature of each request.

Found in the review of #897.