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/series_service_test.go

@@ -26,7 +26,7 @@ func TestContinueSeriesRowToMediaItems_FieldsMappedCorrectly(t *testing.T) {
 		MimeType:       pgtype.Text{String: "application/epub+zip", Valid: true},
 		CoverImagePath: pgtype.Text{String: "/covers/test.jpg", Valid: true},
 		Series:         pgtype.Text{String: "Test Series", Valid: true},
-		SeriesNumber:   pgtype.Int4{Int32: 3, Valid: true},
+		SeriesNumber:   pgtype.Float8{Float64: 3, Valid: true},
 		Tags:           []string{"fantasy", "adventure"},
 		FormatGroup:    "epub",
 		AddedByAdminID: pgtype.UUID{Bytes: adminUUID, Valid: true},
@@ -43,7 +43,7 @@ func TestContinueSeriesRowToMediaItems_FieldsMappedCorrectly(t *testing.T) {
 	assert.Equal(t, pgtype.Text{String: "application/epub+zip", Valid: true}, result.MimeType, "MimeType should match")
 	assert.Equal(t, pgtype.Text{String: "/covers/test.jpg", Valid: true}, result.CoverImagePath, "CoverImagePath should match")
 	assert.Equal(t, pgtype.Text{String: "Test Series", Valid: true}, result.Series, "Series should match")
-	assert.Equal(t, pgtype.Int4{Int32: 3, Valid: true}, result.SeriesNumber, "SeriesNumber should match")
+	assert.Equal(t, pgtype.Float8{Float64: 3, Valid: true}, result.SeriesNumber, "SeriesNumber should match")
 	assert.Equal(t, []string{"fantasy", "adventure"}, result.Tags, "Tags should match")
 	assert.Equal(t, "epub", result.FormatGroup, "FormatGroup should match")
 }
@@ -64,7 +64,7 @@ func TestContinueSeriesRowToMediaItems_NullFieldsHandled(t *testing.T) {
 		MimeType:       pgtype.Text{Valid: false},
 		CoverImagePath: pgtype.Text{Valid: false},
 		Series:         pgtype.Text{Valid: false},
-		SeriesNumber:   pgtype.Int4{Valid: false},
+		SeriesNumber:   pgtype.Float8{Valid: false},
 		Tags:           nil,
 		FormatGroup:    "epub",
 	}
@@ -94,7 +94,7 @@ func TestContinueSeriesRowToMediaItems_AllFieldsMapped(t *testing.T) {
 		MimeType:         pgtype.Text{String: "epub", Valid: true},
 		CoverImagePath:   pgtype.Text{String: "/cover.jpg", Valid: true},
 		Series:           pgtype.Text{String: "Series", Valid: true},
-		SeriesNumber:     pgtype.Int4{Int32: 1, Valid: true},
+		SeriesNumber:     pgtype.Float8{Float64: 1, Valid: true},
 		Tags:             []string{"tag1"},
 		Asin:             pgtype.Text{String: "ASIN", Valid: true},
 		Publisher:        pgtype.Text{String: "Pub", Valid: true},