feat: implement per-user read status tracking

- add user_book_status table (userId, bookId, status, source, timestamps)
- add UserBookStatusModule with repository and service
- auto-update status on progress save (fire-and-forget, skips manual overrides)
- expose PATCH /api/v1/books/:id/status endpoint
- include readStatus on BookCard and BookDetail responses
- surface status icon overlay on cover cards (toggleable)
- add status dropdown in book detail page inline with rating stars
- rewrite statistics summary to query user_book_status directly

client/src/assets/main.css

@@ -218,11 +218,7 @@
   inset: 0;
   z-index: -1;
   pointer-events: none;
-  background: linear-gradient(
-    to bottom,
-    transparent 50%,
-    color-mix(in oklch, var(--primary) 5%, transparent) 50%
-  );
+  background: linear-gradient(to bottom, transparent 50%, color-mix(in oklch, var(--primary) 5%, transparent) 50%);
   background-size: 100% 4px;
 }