build: release a static image with scripts/release.sh

The image now packages a cross-compiled musl binary on distroless instead of
compiling inside Docker. The script builds it, pushes :<version> and :latest
to git.cosocial.info, and tags the commit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

rust-toolchain.toml

@@ -2,3 +2,5 @@
 channel = "1.91.0"
 profile = "minimal"
 components = ["rustfmt", "clippy", "rust-analyzer", "rust-src"]
+# release builds cross-compile a static binary (see scripts/release.sh)
+targets = ["x86_64-unknown-linux-musl"]