cli: survive the --serve transition when nobody is watching

Deploying v0.8.0 broke fmr, and the reason is a flaw I should have seen:
self-update is executed by the OLD binary, so the unit repair I put in the new
binary's updater cannot fix the very update that installs it. The unit kept its
argument-less ExecStart, the new binary answered that with usage and exit 2, and
the service went into a restart loop.

Fixed on fmr by hand, but that is not a fix for anyone else - and the whole
premise of an unattended self-update is that nobody is watching when it happens.

So: when started with no verb *and* systemd started us, the server repairs the
unit and serves anyway, loudly. systemd sets INVOCATION_ID for every service
invocation and nothing else does, so a person at a terminal still gets usage and
a non-zero exit. Marked as a one-release shim to remove once no deployment
predates --serve.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/cmd/echolot-server/main.go

@@ -73,6 +73,26 @@ func run() error {
 
 	switch {
 	case actions.Help:
+		// Compatibility shim for one release.
+		//
+		// Serving became an explicit verb, but self-update is run by the *old* binary — so the
+		// repair added to the updater cannot fix the very update that installs the new one. A
+		// unit written before this change starts us with no arguments, and without this branch
+		// the service would simply stop working, unattended, on a host nobody is watching.
+		//
+		// Only when systemd started us: INVOCATION_ID is set by systemd for every service
+		// invocation and by nothing else, so a person at a terminal still gets usage. Remove
+		// this once no deployment predates --serve.
+		if os.Getenv("INVOCATION_ID") != "" {
+			slog.Warn("started by systemd with no verb — this unit predates --serve; " +
+				"repairing it and serving anyway")
+			if repaired, err := system.RepairExecStart(); err != nil {
+				slog.Error("could not repair the unit; fix ExecStart by hand", "err", err)
+			} else if repaired {
+				slog.Info("systemd unit updated to pass --serve")
+			}
+			return serve(cfg)
+		}
 		config.Usage(os.Stderr)
 		os.Exit(2)
 		return nil