feat: contact form backend for linux and freebsd servers

Assisted-by: GLM 5.3 Flash

docs/CLI.md

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+# Command line
+
+The reference below is taken from the program's own `--help`. If the two
+disagree, the program is right and this file is a defect.
+
+## Synopsis
+
+```sh
+nuntius [--version] [--check-config]
+```
+
+With no flags, nuntius loads the configuration, serves the form endpoints,
+and shuts down gracefully on SIGINT or SIGTERM.
+
+## Global flags
+
+| Flag | Effect |
+|---|---|
+| `--help`, `-h` | prints the usage |
+| `--version` | prints the release and exits: the tag at a tag, a pseudo-version naming the commit otherwise, `(devel)` outside version control, `+dirty` appended on a dirty tree |
+| `--check-config` | loads and validates the configuration, then exits without listening; prints `configuration OK: <path>` on success, names the problem and exits nonzero on any error, so it runs as a systemd `ExecStartPre` |
+
+Every flag is long-only, and Go's `flag` package accepts it with one or two
+leading hyphens, so `-version` and `--version` are the same flag.
+
+## Environment and files
+
+`NUNTIUS_CONFIG` moves the configuration file away from
+`/etc/nuntius/config.toml`. The full schema lives in
+[`CONFIGURATION.md`](CONFIGURATION.md).
+
+## Exit codes
+
+| Code | Meaning |
+|---|---|
+| `0` | the version was printed, the configuration is valid, or the server shut down cleanly |
+| `1` | the configuration is missing, invalid or fails validation, or the server failed to serve |
+
+## Examples
+
+Print the release:
+
+```sh
+nuntius --version
+# nuntius v1.0.0
+```
+
+Validate a configuration without listening, as a deploy pipeline would:
+
+```sh
+NUNTIUS_CONFIG=./config.toml nuntius --check-config
+# configuration OK: ./config.toml
+```
+
+## Manual page
+
+A roff copy of this reference ships as `man/nuntius.1` in the repository
+and installs with the binary; read it with:
+
+```sh
+man ./man/nuntius.1
+```