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/helpers.rs

@@ -0,0 +1,34 @@
+use application::ports::HtmlPageContext;
+use domain::value_objects::UserId;
+
+use crate::state::AppState;
+
+pub(crate) async fn build_page_context(
+    state: &AppState,
+    user_id: Option<UserId>,
+    csrf_token: String,
+) -> HtmlPageContext {
+    let uuid = user_id.as_ref().map(|u| u.value());
+    let (user_email, is_admin) = if let Some(ref id) = user_id {
+        let user = state.app_ctx.repos.user.find_by_id(id).await.ok().flatten();
+        let email = user.as_ref().map(|u| u.email().value().to_string());
+        let admin = user
+            .as_ref()
+            .map(|u| matches!(u.role(), domain::models::UserRole::Admin))
+            .unwrap_or(false);
+        (email, admin)
+    } else {
+        (None, false)
+    };
+    HtmlPageContext {
+        user_email,
+        user_id: uuid,
+        is_admin,
+        register_enabled: state.app_ctx.config.allow_registration,
+        rss_url: "/feed.rss".to_string(),
+        page_title: "Movies Diary".to_string(),
+        canonical_url: state.app_ctx.config.base_url.clone(),
+        csrf_token,
+        page_rss_url: None,
+    }
+}