feat(bundled-binary): release-binaries.yml + install docs + binstall metadata

Adds the tag-triggered workflow that builds 4 musl-linux + macOS
tarballs and attaches them to the tag's GitHub Release. Ships a
matching install guide (docs/install/binary.md) with SHA256SUMS
verify, systemd unit, upgrade flow, and hardware notes. Adds
[package.metadata.binstall] so 'cargo binstall oxicloud' works
automatically once the first release lands.

Also re-enables incremental compilation in the dev profile — the
'modest single-crate savings' rationale from when the crate was small
has been outgrown; full rebuild ~10 min is now the dev-loop bottleneck.

README.md

@@ -68,6 +68,16 @@ docker compose up -d
 
 Open `http://localhost:8086`.
 
+### Prebuilt binary
+
+Binary releases (Linux musl amd64/arm64, macOS Intel/Apple Silicon)
+are attached to every tagged release on GitHub — the whole SPA + all
+operator subcommands + migrations bake into a single self-contained
+executable. See [`docs/install/binary.md`](docs/install/binary.md) for
+the download / verify / systemd walkthrough.
+
+`cargo binstall oxicloud` works too once a release is out.
+
 ### Run from source
 
 Requires Rust 1.93+ and PostgreSQL.