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.

internal/handlers/media.go

@@ -844,7 +844,9 @@ func (mh *MediaHandler) ListMediaItems(c *echo.Context) error {
 			"id":               uuid.UUID(item.ID.Bytes).String(),
 			"library_id":       uuid.UUID(item.LibraryID.Bytes).String(),
 			"title":            item.Title,
+			"subtitle":         utils.PgTextString(item.Subtitle),
 			"author":           utils.PgTextString(item.Author),
+			"authors":          item.Authors,
 			"isbn":             utils.PgTextString(item.Isbn),
 			"description":      utils.PgTextString(item.Description),
 			"file_path":        utils.ResolveMediaURL(item.LibraryID, pgtype.Text{String: item.FilePath, Valid: item.FilePath != ""}),
@@ -857,6 +859,8 @@ func (mh *MediaHandler) ListMediaItems(c *echo.Context) error {
 			"asin":             utils.PgTextString(item.Asin),
 			"date_published":   item.DatePublished.Time.Format("2006-01-02"),
 			"publisher":        utils.PgTextString(item.Publisher),
+			"publishers":       item.Publishers,
+			"explicit":         item.Explicit,
 			"contributors":     item.Contributors,
 			"language":         utils.PgTextString(item.Language),
 			"edition":          utils.PgTextString(item.Edition),
@@ -916,7 +920,9 @@ func (mh *MediaHandler) GetMediaItem(c *echo.Context) error {
 		"id":               uuid.UUID(item.ID.Bytes).String(),
 		"library_id":       uuid.UUID(item.LibraryID.Bytes).String(),
 		"title":            item.Title,
+		"subtitle":         utils.PgTextString(item.Subtitle),
 		"author":           utils.PgTextString(item.Author),
+		"authors":          item.Authors,
 		"isbn":             utils.PgTextString(item.Isbn),
 		"description":      utils.PgTextString(item.Description),
 		"file_path":        utils.ResolveMediaURL(item.LibraryID, pgtype.Text{String: item.FilePath, Valid: item.FilePath != ""}),
@@ -929,6 +935,8 @@ func (mh *MediaHandler) GetMediaItem(c *echo.Context) error {
 		"asin":             utils.PgTextString(item.Asin),
 		"date_published":   item.DatePublished.Time.Format("2006-01-02"),
 		"publisher":        utils.PgTextString(item.Publisher),
+		"publishers":       item.Publishers,
+		"explicit":         item.Explicit,
 		"contributors":     item.Contributors,
 		"language":         utils.PgTextString(item.Language),
 		"edition":          utils.PgTextString(item.Edition),