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/knowledge.service.versions.spec.ts

@@ -5,6 +5,8 @@ import { PrismaService } from "../prisma/prisma.service";
 import { LinkSyncService } from "./services/link-sync.service";
 import { KnowledgeCacheService } from "./services/cache.service";
 import { EmbeddingService } from "./services/embedding.service";
+import { OllamaEmbeddingService } from "./services/ollama-embedding.service";
+import { EmbeddingQueueService } from "./queues/embedding-queue.service";
 import { NotFoundException } from "@nestjs/common";
 
 describe("KnowledgeService - Version History", () => {
@@ -125,6 +127,17 @@ describe("KnowledgeService - Version History", () => {
     batchGenerateEmbeddings: vi.fn().mockResolvedValue([]),
   };
 
+  const mockOllamaEmbeddingService = {
+    isConfigured: vi.fn().mockResolvedValue(false),
+    generateEmbedding: vi.fn().mockResolvedValue([]),
+    generateAndStoreEmbedding: vi.fn().mockResolvedValue(undefined),
+    batchGenerateEmbeddings: vi.fn().mockResolvedValue(0),
+  };
+
+  const mockEmbeddingQueueService = {
+    enqueueEmbeddingGeneration: vi.fn().mockResolvedValue(undefined),
+  };
+
   beforeEach(async () => {
     const module: TestingModule = await Test.createTestingModule({
       providers: [
@@ -145,6 +158,14 @@ describe("KnowledgeService - Version History", () => {
           provide: EmbeddingService,
           useValue: mockEmbeddingService,
         },
+        {
+          provide: OllamaEmbeddingService,
+          useValue: mockOllamaEmbeddingService,
+        },
+        {
+          provide: EmbeddingQueueService,
+          useValue: mockEmbeddingQueueService,
+        },
       ],
     }).compile();
 
@@ -329,7 +350,13 @@ describe("KnowledgeService - Version History", () => {
       // Mock for findVersion call
       mockPrismaService.knowledgeEntry.findUnique.mockResolvedValueOnce(entryWithVersions);
 
-      const result = await service.restoreVersion(workspaceId, slug, 2, userId, "Custom restore note");
+      const result = await service.restoreVersion(
+        workspaceId,
+        slug,
+        2,
+        userId,
+        "Custom restore note"
+      );
 
       expect(result.title).toBe("Test Entry v2");
       expect(result.content).toBe("# Version 2");