feat(#70): implement semantic search API with Ollama embeddings

Updated semantic search to use OllamaEmbeddingService instead of OpenAI:
- Replaced EmbeddingService with OllamaEmbeddingService in SearchService
- Added configurable similarity threshold (SEMANTIC_SEARCH_SIMILARITY_THRESHOLD)
- Updated both semanticSearch() and hybridSearch() methods
- Added comprehensive tests for semantic search functionality
- Updated controller documentation to reflect Ollama requirement
- All tests passing with 85%+ coverage

Related changes:
- Updated knowledge.service.versions.spec.ts to include OllamaEmbeddingService
- Added similarity threshold environment variable to .env.example

Fixes #70

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/api/src/knowledge/search.controller.ts

@@ -101,7 +101,7 @@ export class SearchController {
   /**
    * POST /api/knowledge/search/semantic
    * Semantic search using vector similarity
-   * Requires: Any workspace member, OpenAI API key configured
+   * Requires: Any workspace member, Ollama configured
    *
    * @body query - The search query string (required)
    * @body status - Filter by entry status (optional)