ci(release): verify release provenance for backfill publishes #1465

Job	Run time
gitlawb-core dependency purity	13s
cargo audit	17s
fmt + clippy	43s
release tag validation	4s
MSRV (Rust 1.91)	44s
Docker build smoke test	9m 3s
test (stable)	5m 14s
test (beta)	5m 12s
release versioning coverage	11s
test (windows, non-blocking)	1m 55s
build --release	4m 24s
	28m 0s

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