Docs: privacy and online sharing (#167)

The README gains a Privacy and online sharing section (what Share > Online uploads and where, how long it stays, encrypted in transit but not end-to-end, codes and links are the access, local network sharing is direct and unencrypted, no guarantees), its 'Offline and private' blurb no longer says 'no cloud', and docs/server.md's Security section says the same for operators.

docs/server.md

@@ -667,6 +667,11 @@ sudo systemctl start figcraft-server
 
 ## Security
 
+- **Not end-to-end encrypted.** TLS (from the reverse proxy) protects files on their way to and from
+  the server; on the server they are stored unencrypted, so the operator and anyone who can read
+  the data directory can read every file. Tell the people who publish to your server so (the
+  README's [Privacy and online sharing](../README.md#privacy-and-online-sharing) says it for the
+  default public server), and set `operator_contact` and `terms_url` on a public one.
 - **Access** is a share link (128-bit random token) per file and access level, an owner token
   (128-bit) per file published without the key, or the server key (128-bit) for managing files.
   Tokens are stored as SHA-256 hashes and every comparison runs in constant time; revoking a link