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.

cmd/server/tests/contract_pins_test.go

@@ -662,6 +662,11 @@ func TestContractPin_SearchJSON(t *testing.T) {
 		// EXPLAIN plan-identical) and the search endpoint serializes the
 		// row directly, so they emit here.
 		"abridged", "audio_metadata",
+		// A2b/A2c (2026-10-19): + subtitle, explicit, authors,
+		// authors_search, publishers, publishers_search, narrators_search
+		// (docs/contract_pins.md changelog).
+		"subtitle", "explicit", "authors", "authors_search",
+		"publishers", "publishers_search", "narrators_search",
 	}
 
 	url := fmt.Sprintf("%s/api/media-items/search?q=%s&library_id=%s", setup.Server.URL, url.QueryEscape(fx.itemTitle), fx.libraryID)
@@ -739,10 +744,10 @@ func TestContractPin_MediaListBothBranches(t *testing.T) {
 		"library_type_name_2",
 	}
 	resolvedKeys := []string{
-		"id", "library_id", "title", "author", "isbn", "description",
+		"id", "library_id", "title", "subtitle", "author", "authors", "isbn", "description",
 		"file_path", "file_size", "mime_type", "cover_image_path",
 		"series", "series_number", "tags", "asin", "date_published",
-		"publisher", "contributors", "language", "edition", "page_count",
+		"publisher", "publishers", "explicit", "contributors", "language", "edition", "page_count",
 		"genre", "created_at", "updated_at", "format_group",
 		"manga_type", "reading_direction",
 		"series_count", "volume", "imprint", "age_rating", "web_url",