feat(schema): subtitle/explicit columns, authors+publishers arrays, float series numbers

Phase 1 of the series/collections/metadata overhaul
(~/.opencode/plan/series-collections-metadata-overhaul.md).

Schema (in place, idempotent):
- media_items.series_number INTEGER -> DOUBLE PRECISION: real-world
  ordinals are decimals (0.5, 1.5 novellas); INTEGER silently truncated
  them (calibre 2.5 pinned as 2 in tests).
- + subtitle TEXT, explicit BOOLEAN (sidecar ingest; ABS explicit /
  Audible isAdult).
- + authors/authors_search/publishers/publishers_search TEXT[] NOT NULL
  (narrators A2a precedent). Arrays are write truth; author/publisher
  varchars become trigger-regenerated display strings
  (sync_author_publisher_display) so every varchar consumer (trgm
  search, facets, API, OPDS/Kobo) keeps working unchanged.
- Row-level NULL normalization for the new arrays in
  set_library_type_name (A2 lesson: Go nil -> explicit NULL -> 23502).
- Backfills ARRAY[author]/ARRAY[publisher] — deliberately no
  comma-splitting ('Tolkien, J.R.R.' would corrupt; scanner kept only
  authors[0] historically).
- collection_items.position INTEGER + (collection_id, position) index
  for playlist ordering.

Go: sqlc regen; MediaMetadata/handler params gain Subtitle/Explicit/
Authors/Publishers (legacy singles still accepted, arrays win);
SeriesNumber int32 -> float64 end to end; metadata_overrides adapted
with legacy override keys ('author'/'publisher') preserved so existing
user overrides survive; bulk author edit writes the array.

Pins widened (docs/contract_pins.md): raw sqlc row key set + the six
new columns; changelog entry added.

Verified: unit suite green; integration sweep (bulk/smart collections/
scanner/stats/sync/queue/calibre/pins/audiobooks) green against the
shared dev DB after in-place migration. Pre-existing unrelated red:
TestOwnershipAndAdminTestSuite/TestAdminCreateUser timezone inheritance
fails identically at the pre-change commit (live shared state).

cmd/server/tests/contract_pins_test.go

@@ -123,7 +123,7 @@ func createContractFixture(t *testing.T, setup *TestServerSetup) contractFixture
 	media, err := setup.DB.CreateMediaItem(ctx, database.CreateMediaItemParams{
 		LibraryID: pgtype.UUID{Bytes: libUUID, Valid: true},
 		Title:     itemTitle,
-		Author:    pgtype.Text{String: "Test Contract Author", Valid: true},
+		Authors:  []string{"Test Contract Author"},
 		Series:    pgtype.Text{String: seriesName, Valid: true},
 		Genre:     pgtype.Text{String: "Science Fiction", Valid: true},
 		FilePath:  "/tmp/contract-test-item.epub",
@@ -711,10 +711,14 @@ func TestContractPin_MediaListBothBranches(t *testing.T) {
 	fx := createContractFixture(t, setup)
 
 	// The sqlc row key sets behind BOTH branches.
+	// A2b (2026-10-19): + subtitle, explicit, authors, authors_search,
+	// publishers, publishers_search (docs/contract_pins.md changelog).
 	rawRowKeys := []string{
-		"id", "library_id", "title", "author", "isbn", "description",
+		"id", "library_id", "title", "subtitle", "author", "isbn", "description",
 		"file_path", "file_size", "mime_type", "cover_image_path",
 		"series", "series_number", "tags", "narrators", "abridged",
+		"explicit", "authors", "authors_search", "publishers",
+		"publishers_search",
 		"asin", "date_published", "publisher", "contributors", "language",
 		"edition", "page_count", "genre", "copyright_year", "goodreads_id",
 		"openlibrary_id", "google_books_id", "added_by_admin_id", "created_at",