chore: Add deploy plumbing for the share service

Add a hardened systemd unit, a Caddy reverse-proxy snippet that maps
/fabula/api/* to the loopback service, and Justfile build-server/
deploy-server recipes that build a static binary and ship + restart it.
Includes server/README documenting the API, config, and deploy steps.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/Caddyfile.snippet

@@ -0,0 +1,21 @@
+# Caddy config for the share-link service.
+#
+# The frontend (served under /fabula/) calls /fabula/api/s, but the Go service
+# routes plain /api/s. `uri strip_prefix /fabula` rewrites /fabula/api/s ->
+# /api/s before proxying to the service on loopback.
+#
+# Place this INSIDE your existing site block, and make sure this /fabula/api/*
+# handler comes BEFORE the static handler that serves /fabula/* — `handle`
+# blocks match in source order, first match wins.
+
+handle /fabula/api/* {
+	uri strip_prefix /fabula
+	reverse_proxy 127.0.0.1:8090
+}
+
+# ... your existing static file handler for /fabula/* follows here, e.g.:
+#
+# handle /fabula/* {
+# 	root * /usr/share/caddy/public_html
+# 	file_server
+# }