Encrypt pages at rest with a key per person

Pages are sealed with AES-256-GCM. Each person's key is derived (HKDF) from a master key in the settings file plus their Authentik issuer and subject. Plain files from earlier versions are read and re-saved encrypted. Mako refuses to open a notebook it cannot decrypt rather than overwrite it.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

deploy/install.sh

@@ -155,7 +155,8 @@ MAKO_OIDC_CLIENT_SECRET=
 # everyone Authentik lets into the application gets a private notebook of their own.
 #MAKO_ALLOWED=you@example.com
 
-# Where the settings file is (it holds the signing secret).
+# Where the settings file is. It holds the signing secret and the key that encrypts your notes:
+# back it up, apart from $DATA_DIR. Without that key the notes cannot be read.
 MAKO_CONFIG=$CONF_FILE
 
 # Optional. These override what the settings file says.