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_metadata_test.go

@@ -139,8 +139,8 @@ func TestParseOPFContentCalibreSeries(t *testing.T) {
 	if err != nil {
 		t.Fatalf("parseOPFContent() error: %v", err)
 	}
-	if metadata.Series != "Great Series" || metadata.SeriesNumber != 2 {
-		t.Errorf("Series = %q/%d, want Great Series/2", metadata.Series, metadata.SeriesNumber)
+	if metadata.Series != "Great Series" || metadata.SeriesNumber != 2.5 {
+		t.Errorf("Series = %q/%v, want Great Series/2.5", metadata.Series, metadata.SeriesNumber)
 	}
 	if metadata.ISBN == "" {
 		t.Error("schemed ISBN not extracted")
@@ -179,8 +179,8 @@ func TestExtractAudiobookshelfSidecar(t *testing.T) {
 				if m.Title != "An Book" || m.Author != "Author One" {
 					t.Errorf("Title/Author = %q/%q", m.Title, m.Author)
 				}
-				if m.Series != "The Series" || m.SeriesNumber != 4 {
-					t.Errorf("Series = %q/%d, want The Series/4", m.Series, m.SeriesNumber)
+				if m.Series != "The Series" || m.SeriesNumber != 4.5 {
+					t.Errorf("Series = %q/%v, want The Series/4.5", m.Series, m.SeriesNumber)
 				}
 				if len(m.Tags) != 2 {
 					t.Errorf("Tags = %v, want genres+tags merged", m.Tags)