Allow server/client-side encryption of chats

Feature request

Chats should be stored encrypted with user/server keys.

Ideally this should include:

• encryption of text message content so that it is not stored in plaintext in the database;
• an architecture where the server cannot trivially read message contents;
• proper key handling for users/devices;
• support for message history in a secure way;
• clear documentation about the security model and limitations.

Current drawbacks

• Message search is relying on plain text and will be unavailable / problematic to implement

Possible solutions

• Create some chats as encrypted, warning that message search will not work for these messages

See also:

• #37 for E2E (client side) encryption
• https://github.com/nextcloud/contacts/issues/274 similar request for contacts
• https://github.com/nextcloud/server/issues/5927 similar request for calendar
• https://github.com/nextcloud/server/issues/13103 general request for encrypting parts of database

No files, tests, or entry points are named. Start by reviewing the existing encryption discussion in #37 and the related security requests, then define the client/server key-handling and message-history boundaries. Done should include an agreed security model, encrypted storage behavior, documented limitations, and an explicit decision about message search.