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

.env.example

@@ -88,6 +88,14 @@ JWT_EXPIRATION=24h
 OLLAMA_ENDPOINT=http://ollama:11434
 OLLAMA_PORT=11434
 
+# ======================
+# OpenAI API (For Semantic Search)
+# ======================
+# OPTIONAL: Semantic search requires an OpenAI API key
+# Get your API key from: https://platform.openai.com/api-keys
+# If not configured, semantic search endpoints will return an error
+# OPENAI_API_KEY=sk-...
+
 # ======================
 # Application Environment
 # ======================