refactor: split monolithic handlers + testing into domain-grouped modules

handlers/api.rs (1706 LOC) + html.rs (1735 LOC) → 12 domain files:
auth, diary, movies, users, search, watchlist, goals, social,
integrations, helpers + existing import/webhook/wrapup/images/rss.

domain/testing.rs (1309 LOC) → testing/ module:
in_memory, fakes, noops, panics, wrapup.

Update README + architecture.mmd with goals feature.

crates/presentation/src/openapi/social.rs

@@ -10,20 +10,20 @@ use utoipa::OpenApi;
 #[derive(OpenApi)]
 #[openapi(
     paths(
-        crate::handlers::api::get_following,
-        crate::handlers::api::get_followers,
-        crate::handlers::api::get_pending_followers,
-        crate::handlers::api::follow,
-        crate::handlers::api::unfollow,
-        crate::handlers::api::accept_follower,
-        crate::handlers::api::reject_follower,
-        crate::handlers::api::remove_follower,
-        crate::handlers::api::get_blocked_domains_admin,
-        crate::handlers::api::add_blocked_domain_admin,
-        crate::handlers::api::remove_blocked_domain_admin,
-        crate::handlers::api::block_actor_api,
-        crate::handlers::api::unblock_actor_api,
-        crate::handlers::api::get_blocked_actors_api,
+        crate::handlers::social::get_following,
+        crate::handlers::social::get_followers,
+        crate::handlers::social::get_pending_followers,
+        crate::handlers::social::follow,
+        crate::handlers::social::unfollow,
+        crate::handlers::social::accept_follower,
+        crate::handlers::social::reject_follower,
+        crate::handlers::social::remove_follower,
+        crate::handlers::social::get_blocked_domains_admin,
+        crate::handlers::social::add_blocked_domain_admin,
+        crate::handlers::social::remove_blocked_domain_admin,
+        crate::handlers::social::block_actor_api,
+        crate::handlers::social::unblock_actor_api,
+        crate::handlers::social::get_blocked_actors_api,
     ),
     components(schemas(
         ActorListResponse,