Remove federation functionality and related tests

- Deleted the `federation.rs` module and its associated functionality for federating thoughts to followers.
- Removed the `well_known.rs` module and its WebFinger discovery functionality.
- Eliminated references to federation in the `thought.rs` router and removed the spawning of background tasks for federating thoughts.
- Deleted tests related to WebFinger and user inbox interactions in `activitypub.rs`.
- Updated `Cargo.toml` to remove the `activitypub_federation` dependency.

thoughts-backend/tests/api/mod.rs

@@ -1,4 +1,3 @@
-mod activitypub;
 mod api_key;
 mod auth;
 mod feed;