feat(mangas): add tag-similarity "Similar" section on manga detail

Add GET /v1/mangas/:id/similar returning the top 5 mangas ranked by
Jaccard tag overlap (shared / union), excluding self, as MangaCard items
in a plain { items } object. Surface them in a hidden-when-empty
"Similar" section on the manga detail page, reusing MangaCard.

Backend: new repo::manga::list_similar with a manga_tags self-join; a
shared cards_from_rows helper (also used by list_cards) that loads
authors+genres concurrently; single-sourced column list via MANGA_COLS +
manga_cols(alias) to replace the fragile SELECT_COLS string-splitting.

Frontend: getSimilarMangas client fn (guards a malformed body), loader
fetch with non-critical .catch fallback, and the catalog grid hoisted to
a shared global .manga-grid (lib/styles/tokens.css), de-duplicating the
per-route copies.

Bump version 0.62.0 -> 0.63.0 (backend + frontend in lockstep).

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

frontend/src/routes/collections/[id]/+page.svelte

@@ -314,14 +314,7 @@
         color: var(--text-muted);
     }
 
-    .manga-grid {
-        list-style: none;
-        padding: 0;
-        margin: 0;
-        display: grid;
-        grid-template-columns: repeat(auto-fill, minmax(140px, 1fr));
-        gap: var(--space-4);
-    }
+    /* `.manga-grid` is a shared global (see lib/styles/tokens.css). */
 
     .section-heading {
         margin: var(--space-5) 0 var(--space-3);