docs: make static deploy cache-safe

deploy/Caddyfile.snippet

@@ -12,6 +12,10 @@
 #     tera.lumbridgecorp.com {
 #         import tera_api
 #         root * /srv/tera/dist
+#         @assets path /assets/*
+#         header @assets Cache-Control "public, max-age=31536000, immutable"
+#         @documents not path /assets/*
+#         header @documents Cache-Control "no-cache"
 #         file_server
 #     }
 #
@@ -32,6 +36,10 @@
 #     office.lumbridgecorp.com {
 #         import tera_api
 #         root * /srv/tera/dist          # the SAME root as tera., not a copy
+#         @assets path /assets/*
+#         header @assets Cache-Control "public, max-age=31536000, immutable"
+#         @documents not path /assets/*
+#         header @documents Cache-Control "no-cache"
 #         try_files {path} {path}/index.html /office.html
 #         file_server
 #     }
@@ -42,6 +50,10 @@
 # either. The only line that differs between the two site blocks is the
 # `try_files` fallback.
 #
+# Keep the cache rules in the static handler when the site also serves the API.
+# The HTML shells and RELEASE_SHA are stable names and must revalidate after a
+# deploy; Vite's /assets/* files are content-addressed and may stay immutable.
+#
 # A deployment that skips this is not broken: the office door keeps working and
 # simply unfurls with the city's card, which is what it did before there was a
 # second shell at all.