Extend dismissed-exclusion to mood feeds for consistency

Mood feeds now honor the same dismissed list as the brief: /api/feed accepts an
exclude param (over-fetching to stay full), and the client passes the persisted
dismissed set. Swapping a story away now keeps it gone everywhere — brief and
browse — not just on the home view. Also simplified the feed filter path to the
shared _prefs_sql_kw helper.

Tests: feed exclude (91 total).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/+page.svelte

@@ -83,7 +83,8 @@
       } else {
         const mood = moods.find((m) => m.key === key);
         const q = P.param(P.merge(userPrefs, mood?.filter ?? {}));
-        feed = (await getJSON(`/api/feed?limit=24${q ? '&' + q : ''}`)).items;
+        const ex = Array.from(dismissed).join(',');
+        feed = (await getJSON(`/api/feed?limit=24${q ? '&' + q : ''}${ex ? '&exclude=' + ex : ''}`)).items;
         remember(feed);
       }
     } catch (e) {