fix: add missing opposite relation fields for Prisma validation

- KnowledgeBase.folders ← KnowledgeFolder.knowledgeBase
- KnowledgeSource.references ← SourceReference.source
- KnowledgeChunk.references ← SourceReference.chunk

Fixes P1012 Prisma schema validation errors in CI build.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

prisma/schema.prisma

@@ -179,6 +179,7 @@ model KnowledgeBase {
   candidates    ImportCandidate[]
   chunks        KnowledgeChunk[]
   focusItems    FocusItem[]
+  folders       KnowledgeFolder[]
 
   @@index([userId])
   @@index([status])
@@ -701,6 +702,7 @@ model KnowledgeSource {
   file              UploadedFile? @relation(fields: [fileId], references: [id])
   chunks            KnowledgeChunk[]
   imports           DocumentImport[]
+  references        SourceReference[]
   candidates        ImportCandidate[]
 
   @@index([userId])
@@ -731,6 +733,7 @@ model KnowledgeChunk {
   user             User      @relation(fields: [userId], references: [id])
   knowledgeBase    KnowledgeBase @relation(fields: [knowledgeBaseId], references: [id])
   source           KnowledgeSource @relation(fields: [sourceId], references: [id])
+  references       SourceReference[]
 
   @@index([userId])
   @@index([sourceId])