perf: optimise la query des auteurs — single pass + index GIN

- Remplace les 5 CTEs + double query (données + count) par une seule
  requête avec COUNT(*) OVER() pour le total
- Calcule book_count et series_count directement depuis UNNEST, sans
  re-JOIN sur les tables
- Ajoute des index GIN sur books.authors et series_metadata.authors

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

infra/migrations/0066_add_authors_index.sql

@@ -0,0 +1,8 @@
+-- GIN index for efficient array lookups on books.authors
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_books_authors_gin ON books USING GIN (authors);
+
+-- Index on author column for legacy single-author field
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_books_author ON books (author) WHERE author IS NOT NULL AND author != '';
+
+-- GIN index on series_metadata.authors
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_series_metadata_authors_gin ON series_metadata USING GIN (authors);