feat(filters+nav): author navigation end-to-end — arrays on every media response, deep-linkable bookshelf filters, clickable authors

Completes the A2c acceptance scenario: click an author on a book
detail page -> land on the bookshelf filtered to everything that
author wrote (including their share of co-authored books).

- Media detail + unscoped list resolved maps expose subtitle,
  explicit, authors, publishers (the library-scoped raw branch and
  the search endpoint already did — raw rows follow the SELECT).
- SearchMediaItemsUnified SELECT carries the new columns; the search
  endpoint serializes the row directly, so clients get them there too.
- book_detail.templ renders each author as a link to
  /bookshelf?author_filter=<name>.
- Bookshelf honors filter params in the URL on load (author_filter,
  series_filter, genre_filter, publisher_filter, tags_filter,
  narrator_filter, language_filter): pre-fills the form, applies it to
  the first grid load. Comma-joined values are multi-select via the
  server-side string_to_array split. Filtered views are shareable.

Pins widened: search rows (+7 keys), resolved keys (+4); changelog
entries in docs/contract_pins.md. Android follow-up plan written to
~/.opencode/plan/bookhoard-app-v2-compat.md.

docs/contract_pins.md

@@ -152,3 +152,12 @@ cover_image_path, has_conflict, format_group, badge`.
 - User collections are dashboard-eligible by default (the
   show_on_dashboard gate is gone from GetUserCollectionsForDashboard);
   hiding stays dashboard-prefs (`hidden_collections`) territory.
+- `GET /api/media-items` (unscoped resolved-map branch) and
+  `GET /api/media-items/:id` gained the same ADDITIVE keys as the raw
+  branch: `subtitle`, `explicit`, `authors`, `publishers`.
+- `GET /api/media-items/search` rows gained the same ADDITIVE keys plus
+  the search pairs `authors_search`, `publishers_search`,
+  `narrators_search` (the unified SELECT now carries the new columns).
+- `author_filter` / `publisher_filter` are now per-element against the
+  arrays; a comma-joined value is OR-within-facet multi-select. The
+  param NAMES and single-value behavior are unchanged.