feat: add semantic search with pgvector (closes #68, #69, #70)

Issues resolved:
- #68: pgvector Setup
  * Added pgvector vector index migration for knowledge_embeddings
  * Vector index uses HNSW algorithm with cosine distance
  * Optimized for 1536-dimension OpenAI embeddings

- #69: Embedding Generation Pipeline
  * Created EmbeddingService with OpenAI integration
  * Automatic embedding generation on entry create/update
  * Batch processing endpoint for existing entries
  * Async generation to avoid blocking API responses
  * Content preparation with title weighting

- #70: Semantic Search API
  * POST /api/knowledge/search/semantic - pure vector search
  * POST /api/knowledge/search/hybrid - RRF combined search
  * POST /api/knowledge/embeddings/batch - batch generation
  * Comprehensive test coverage
  * Full documentation in docs/SEMANTIC_SEARCH.md

Technical details:
- Uses OpenAI text-embedding-3-small model (1536 dims)
- HNSW index for O(log n) similarity search
- Reciprocal Rank Fusion for hybrid search
- Graceful degradation when OpenAI not configured
- Async embedding generation for performance

Configuration:
- Added OPENAI_API_KEY to .env.example
- Optional feature - disabled if API key not set
- Falls back to keyword search in hybrid mode

apps/api/src/knowledge/knowledge.module.ts

@@ -2,7 +2,11 @@ import { Module } from "@nestjs/common";
 import { PrismaModule } from "../prisma/prisma.module";
 import { AuthModule } from "../auth/auth.module";
 import { KnowledgeService } from "./knowledge.service";
-import { KnowledgeController, KnowledgeCacheController } from "./knowledge.controller";
+import {
+  KnowledgeController,
+  KnowledgeCacheController,
+  KnowledgeEmbeddingsController,
+} from "./knowledge.controller";
 import { SearchController } from "./search.controller";
 import { KnowledgeStatsController } from "./stats.controller";
 import {
@@ -12,6 +16,7 @@ import {
   GraphService,
   StatsService,
   KnowledgeCacheService,
+  EmbeddingService,
 } from "./services";
 
 @Module({
@@ -19,6 +24,7 @@ import {
   controllers: [
     KnowledgeController,
     KnowledgeCacheController,
+    KnowledgeEmbeddingsController,
     SearchController,
     KnowledgeStatsController,
   ],
@@ -30,7 +36,8 @@ import {
     GraphService,
     StatsService,
     KnowledgeCacheService,
+    EmbeddingService,
   ],
-  exports: [KnowledgeService, LinkResolutionService, SearchService],
+  exports: [KnowledgeService, LinkResolutionService, SearchService, EmbeddingService],
 })
 export class KnowledgeModule {}