Add Linux deployment installer, supervised services and readiness checks

deploy/linux/api.env.example

@@ -0,0 +1,3 @@
+# Read by systemd as root, not by the web user. chmod 0600, owner root.
+# Configure once; never commit a filled copy.
+# VBA_INTAKE_TOKEN="replace-with-a-random-32-to-256-character-base64url-token"