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/domain/src/testing/mod.rs

@@ -0,0 +1,11 @@
+mod fakes;
+mod in_memory;
+mod noops;
+mod panics;
+mod wrapup;
+
+pub use fakes::*;
+pub use in_memory::*;
+pub use noops::*;
+pub use panics::*;
+pub use wrapup::*;