Bound what a request can make the server hold

Cap JSON bodies at 64 KB on every API route except JMAP and uploads, which
bound themselves. Sign-in used to read a body of any size before its rate
limits ran; the flood ceiling now also runs before the body is read.

For sessions whose JMAP requests are checked, lower the read cap from 16 MB
to 4 MB, allow four such reads per session at once, and turn requests away
with a 503 once 32 MB is held across everyone.

Count sign-in limits per /64 for IPv6, since one host holds a whole /64.

Bind the compose example to loopback, and run it read-only with no
capabilities and no-new-privileges. Keep .env.* out of git and the image
build context.

.dockerignore

@@ -3,4 +3,7 @@ node_modules
 **/dist
 .git
 .env
+# deploy.example.sh keeps its settings in .env.production; any .env.* holds APP_SECRET.
+.env.*
+!.env.example
 server/data