feat(admin): Implement System Knowledge Base management module

Features:

- Backend: SystemKbService with full CRUD (knowledge bases + documents)

- Backend: 8 RESTful API endpoints (list/detail/create/update/delete/upload/download)

- Backend: OSS storage integration (system/knowledge-bases/{kbId}/{docId})

- Backend: RAG engine integration (document parsing, chunking, vectorization)

- Frontend: SystemKbListPage with card-based layout

- Frontend: SystemKbDetailPage with document management table

- Frontend: Master-Detail UX pattern for better user experience

- Document upload (single/batch), download (preserving original filename), delete

Technical:

- Database migration for system_knowledge_bases and system_kb_documents tables

- OSSAdapter.getSignedUrl with Content-Disposition for original filename

- Reuse RAG engine from common/rag for document processing

Tested: Local environment verified, all features working

backend/verify-migration.cjs

@@ -0,0 +1,22 @@
+const { PrismaClient } = require('@prisma/client');
+const prisma = new PrismaClient();
+
+async function main() {
+  const tables = await prisma.$queryRawUnsafe(`
+    SELECT table_name FROM information_schema.tables 
+    WHERE table_schema = 'capability_schema' ORDER BY table_name
+  `);
+  console.log('=== capability_schema Tables ===');
+  tables.forEach(t => console.log('-', t.table_name));
+  
+  const cols = await prisma.$queryRawUnsafe(`
+    SELECT column_name FROM information_schema.columns 
+    WHERE table_schema = 'capability_schema' AND table_name = 'prompt_templates'
+    ORDER BY ordinal_position
+  `);
+  console.log('\n=== prompt_templates Columns ===');
+  cols.forEach(c => console.log('-', c.column_name));
+  
+  await prisma.$disconnect();
+}
+main();