fix(plugins): unique csv row keys, per-file default mediaType, scale-aware zero rating

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

plugins/csv/src/mapping.ts

@@ -115,13 +115,14 @@ export function defaultScale(rows: FieldRow[]): 5 | 10 | 100 {
   return max <= 5 ? 5 : max <= 10 ? 10 : 100;
 }
 
-/** Returns a 0-100 rating, or null when unrated. 0 counts as unrated (Goodreads convention). */
+/** Returns a 0-100 rating, or null when unrated. 0 counts as unrated only on scale 5 (Goodreads convention). */
 export function normalizeRating(r: FieldRow, fileScale: 5 | 10 | 100): number | null {
   if (r.rating === undefined) return null;
   const n = Number(r.rating);
-  if (!Number.isFinite(n) || n <= 0) return null;
+  if (!Number.isFinite(n) || n < 0) return null;
   const rs = Number(r.ratingScale);
   const scale = rs === 5 || rs === 10 || rs === 100 ? rs : fileScale;
+  if (n === 0 && scale === 5) return null;
   return Math.max(0, Math.min(100, Math.round((n / scale) * 100)));
 }