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/users.rs

@@ -7,11 +7,11 @@ use utoipa::OpenApi;
 #[derive(OpenApi)]
 #[openapi(
     paths(
-        crate::handlers::api::list_users,
-        crate::handlers::api::get_user_profile,
-        crate::handlers::api::get_profile,
-        crate::handlers::api::update_profile_handler,
-        crate::handlers::api::update_profile_fields_handler,
+        crate::handlers::users::list_users,
+        crate::handlers::users::get_user_profile,
+        crate::handlers::users::get_profile,
+        crate::handlers::users::update_profile_handler,
+        crate::handlers::users::update_profile_fields_handler,
     ),
     components(schemas(
         UsersResponse,