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).

internal/services/media_scanner_audio_multifile.go

@@ -536,7 +536,9 @@ func (s *MediaScanner) createAudioBookItem(ctx context.Context, libraryID pgtype
 	return s.db.CreateMediaItem(ctx, database.CreateMediaItemParams{
 		LibraryID:       libraryID,
 		Title:           metadata.Title,
-		Author:          pgtype.Text{String: metadata.Author, Valid: metadata.Author != ""},
+		Subtitle:        pgtype.Text{String: metadata.Subtitle, Valid: metadata.Subtitle != ""},
+		Authors:         nilToEmpty(utils.SingleOrSlice(metadata.Author, metadata.Authors)),
+		AuthorsSearch:   utils.NormalizeTagsSearch(utils.SingleOrSlice(metadata.Author, metadata.Authors)),
 		Isbn:            pgtype.Text{String: utils.NormalizeISBNSafe(metadata.ISBN), Valid: metadata.ISBN != ""},
 		Asin:            pgtype.Text{String: metadata.ASIN, Valid: metadata.ASIN != ""},
 		Description:     pgtype.Text{String: metadata.Description, Valid: metadata.Description != ""},
@@ -545,14 +547,16 @@ func (s *MediaScanner) createAudioBookItem(ctx context.Context, libraryID pgtype
 		MimeType:        pgtype.Text{String: mimeType, Valid: mimeType != ""},
 		CoverImagePath:  pgtype.Text{String: metadata.CoverPath, Valid: metadata.CoverPath != ""},
 		Series:          pgtype.Text{String: metadata.Series, Valid: metadata.Series != ""},
-		SeriesNumber:    pgtype.Int4{Int32: metadata.SeriesNumber, Valid: metadata.SeriesNumber > 0},
-		Publisher:       pgtype.Text{String: metadata.Publisher, Valid: metadata.Publisher != ""},
+		SeriesNumber:    pgtype.Float8{Float64: metadata.SeriesNumber, Valid: metadata.SeriesNumber > 0},
+		Publishers:      nilToEmpty(utils.SingleOrSlice(metadata.Publisher, metadata.Publishers)),
+		PublishersSearch: utils.NormalizeTagsSearch(utils.SingleOrSlice(metadata.Publisher, metadata.Publishers)),
 		DatePublished:   pgtype.Date{Time: metadata.PublishDate, Valid: !metadata.PublishDate.IsZero()},
 		Genre:           pgtype.Text{String: metadata.Genre, Valid: metadata.Genre != ""},
 		Contributors:    utils.NormalizeContributors(metadata.Contributors),
 		Narrators:       nilToEmpty(metadata.Narrators),
 		NarratorsSearch: utils.NormalizeNarratorsSearch(metadata.Narrators),
 		Abridged:        metadata.Abridged,
+		Explicit:        metadata.Explicit,
 		Tags:            utils.NormalizeTags(metadata.Tags),
 		AddedByAdminID:  s.adminID,
 		CreatedAt:       modTime,