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/handlers/mod.rs

@@ -1,8 +1,17 @@
-pub mod api;
-pub mod html;
+pub mod auth;
+pub mod diary;
+pub mod goals;
+mod helpers;
 pub mod images;
 pub mod import;
+pub mod integrations;
+pub mod movies;
 pub mod rss;
+pub mod search;
+#[cfg(feature = "federation")]
+pub mod social;
+pub mod users;
+pub mod watchlist;
 pub mod webhook;
 pub mod wrapup;