Finish implementation of basic character functionality

This issue is for tracking all the pending character creation/editing/listing functionality implementations. Current task list:

[x] (b196c6268188b7888b1bf4b102c998fa35098906) For endpoints that return more than one character, serialize only the minimal set of fields that will likely be required by the front-end
- For example, world_scenario, greeting, persona and so on don't need to be sent to the FE when just browsing through characters.
[x] (2bec8b64f27a3d4e772ffcc3cc58ea29b3097f4f) Only allow characters to be edited by their creators and/or admins
- As of now no checks are in place, so if you know a character's UUID you can edit it.
[x] (b196c6268188b7888b1bf4b102c998fa35098906) Handle unlisted and private bots correctly
- For any endpoints that return characters, they should be filtered so only public ones are shown, plus the ones created by the currently logged-in user.
[x] (38dbb8424327cc1e255dd9a29f24ab8d4ab49ff3) Replace hard delete with soft deletion instead
- That way we won't have any data integrity problems.

PygmalionAI / paphos-backend