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.controller.ts

@@ -12,6 +12,7 @@ import {
   DefaultValuePipe,
 } from "@nestjs/common";
 import type { AuthUser } from "@mosaic/shared";
+import { EntryStatus } from "@prisma/client";
 import { KnowledgeService } from "./knowledge.service";
 import { CreateEntryDto, UpdateEntryDto, EntryQueryDto, RestoreVersionDto } from "./dto";
 import { AuthGuard } from "../auth/guards/auth.guard";
@@ -192,6 +193,38 @@ export class KnowledgeController {
   }
 }
 
+/**
+ * Controller for knowledge embeddings endpoints
+ */
+@Controller("knowledge/embeddings")
+@UseGuards(AuthGuard, WorkspaceGuard, PermissionGuard)
+export class KnowledgeEmbeddingsController {
+  constructor(private readonly knowledgeService: KnowledgeService) {}
+
+  /**
+   * POST /api/knowledge/embeddings/batch
+   * Batch generate embeddings for all entries in the workspace
+   * Useful for populating embeddings for existing entries
+   * Requires: ADMIN role or higher
+   */
+  @Post("batch")
+  @RequirePermission(Permission.WORKSPACE_ADMIN)
+  async batchGenerate(
+    @Workspace() workspaceId: string,
+    @Body() body: { status?: string }
+  ) {
+    const status = body.status as EntryStatus | undefined;
+    const result = await this.knowledgeService.batchGenerateEmbeddings(
+      workspaceId,
+      status
+    );
+    return {
+      message: `Generated ${result.success} embeddings out of ${result.total} entries`,
+      ...result,
+    };
+  }
+}
+
 /**
  * Controller for knowledge cache endpoints
  */