feat: signed releases, release feed, notification (read-only)

docs/deployment.md

@@ -19,8 +19,22 @@ task build     # this OS/arch → bin/renewcms[.exe]
 task release   # all targets  → bin/renewcms_<version>_<os>_<arch>[.exe]
 ```
 
+`task release` also signs the release: it writes `bin/checksums.txt` (SHA-256 of the six binaries) and signs it into `bin/checksums.txt.sig` with an Ed25519 key. It refuses to start, before building anything, unless `RELEASE_SIGNING_KEY` points to the private key file and that key matches the public key committed as `internal/infrastructure/update/release.pub`. The one-time setup is below.
+
 The frontend is built once and embedded into every binary. The binaries are static (`CGO_ENABLED=0`), so a Linux build runs on any distribution. The version comes from `git describe`: tag a release (`git tag v1.2.0`) before building, and the binary logs its version at startup.
 
+### Release signing key (maintainers, once)
+
+1. Run `task release:keygen -- <path>` with a path outside the repository (for example `~/.renewcms/release.key`). The private key is written there and never leaves your machine; keygen refuses a path inside the work tree.
+2. Commit the printed public key as `internal/infrastructure/update/release.pub`. Until you do, the file holds a placeholder (32 zero bytes) and `task release` fails with a message saying so.
+3. Export `RELEASE_SIGNING_KEY=<path to the private key>` before every `task release`.
+
+To check a download by hand: `go run ./cmd/releasesign verify checksums.txt`, with `checksums.txt.sig` next to it, then compare the binary's hash with the matching line of `checksums.txt`.
+
+### Update checks
+
+The owner's Settings page asks the release feed for the newest release (see `UPDATE_FEED_URL` in [configuration.md](configuration.md)) and keeps the last result in `update-status.json`, next to `DB_FILE`. Back it up with the rest of the data directory if you like; it is rebuilt by the next check when missing. The Settings page only reports a newer version for now: the site does not update itself.
+
 ## 2. Lay out the server
 
 The following is a suggested layout for Linux: