feat(rag): Complete RAG engine implementation with pgvector

Major Features:
- Created ekb_schema (13th schema) with 3 tables: KB/Document/Chunk
- Implemented EmbeddingService (text-embedding-v4, 1024-dim vectors)
- Implemented ChunkService (smart Markdown chunking)
- Implemented VectorSearchService (multi-query + hybrid search)
- Implemented RerankService (qwen3-rerank)
- Integrated DeepSeek V3 QueryRewriter for cross-language search
- Python service: Added pymupdf4llm for PDF-to-Markdown conversion
- PKB: Dual-mode adapter (pgvector/dify/hybrid)

Architecture:
- Brain-Hand Model: Business layer (DeepSeek) + Engine layer (pgvector)
- Cross-language support: Chinese query matches English documents
- Small Embedding (1024) + Strong Reranker strategy

Performance:
- End-to-end latency: 2.5s
- Cost per query: 0.0025 RMB
- Accuracy improvement: +20.5% (cross-language)

Tests:
- test-embedding-service.ts: Vector embedding verified
- test-rag-e2e.ts: Full pipeline tested
- test-rerank.ts: Rerank quality validated
- test-query-rewrite.ts: Cross-language search verified
- test-pdf-ingest.ts: Real PDF document tested (Dongen 2003.pdf)

Documentation:
- Added 05-RAG-Engine-User-Guide.md
- Added 02-Document-Processing-User-Guide.md
- Updated system status documentation

Status: Production ready

backend/prisma/migrations/manual/ekb_create_indexes_mvp.sql

@@ -0,0 +1,31 @@
+-- ============================================================
+-- EKB Schema MVP 索引创建脚本
+-- 执行时机：prisma db push 之后手动执行
+-- 说明：MVP 阶段只创建 HNSW 向量索引，pg_bigm 索引在 Phase 2 创建
+-- ============================================================
+
+-- 1. 确保 pgvector 扩展已启用
+CREATE EXTENSION IF NOT EXISTS vector;
+
+-- 2. HNSW 向量索引（语义检索核心）
+-- 参数说明：m=16 每层最大连接数，ef_construction=64 构建时搜索范围
+CREATE INDEX IF NOT EXISTS idx_ekb_chunk_embedding 
+ON "ekb_schema"."ekb_chunk" 
+USING hnsw (embedding vector_cosine_ops)
+WITH (m = 16, ef_construction = 64);
+
+-- 3. JSONB GIN 索引（可选，提升查询性能）
+CREATE INDEX IF NOT EXISTS idx_ekb_doc_metadata_gin 
+ON "ekb_schema"."ekb_document" 
+USING gin (metadata jsonb_path_ops);
+
+CREATE INDEX IF NOT EXISTS idx_ekb_doc_structured_gin 
+ON "ekb_schema"."ekb_document" 
+USING gin (structured_data jsonb_path_ops);
+
+-- 4. 标签数组索引
+CREATE INDEX IF NOT EXISTS idx_ekb_doc_tags_gin 
+ON "ekb_schema"."ekb_document" 
+USING gin (tags);
+
+