feat(#71): implement graph data API

Implemented three new API endpoints for knowledge graph visualization:

1. GET /api/knowledge/graph - Full knowledge graph
   - Returns all entries and links with optional filtering
   - Supports filtering by tags, status, and node count limit
   - Includes orphan detection (entries with no links)

2. GET /api/knowledge/graph/stats - Graph statistics
   - Total entries and links counts
   - Orphan entries detection
   - Average links per entry
   - Top 10 most connected entries
   - Tag distribution across entries

3. GET /api/knowledge/graph/:slug - Entry-centered subgraph
   - Returns graph centered on specific entry
   - Supports depth parameter (1-5) for traversal distance
   - Includes all connected nodes up to specified depth

New Files:
- apps/api/src/knowledge/graph.controller.ts
- apps/api/src/knowledge/graph.controller.spec.ts

Modified Files:
- apps/api/src/knowledge/dto/graph-query.dto.ts (added GraphFilterDto)
- apps/api/src/knowledge/entities/graph.entity.ts (extended with new types)
- apps/api/src/knowledge/services/graph.service.ts (added new methods)
- apps/api/src/knowledge/services/graph.service.spec.ts (added tests)
- apps/api/src/knowledge/knowledge.module.ts (registered controller)
- apps/api/src/knowledge/dto/index.ts (exported new DTOs)
- docs/scratchpads/71-graph-data-api.md (implementation notes)

Test Coverage: 21 tests (all passing)
- 14 service tests including orphan detection, filtering, statistics
- 7 controller tests for all three endpoints

Follows TDD principles with tests written before implementation.
All code quality gates passed (lint, typecheck, tests).

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

apps/api/src/knowledge/dto/graph-query.dto.ts

@@ -1,5 +1,6 @@
-import { IsOptional, IsInt, Min, Max } from "class-validator";
+import { IsOptional, IsInt, Min, Max, IsString, IsEnum, IsArray } from "class-validator";
 import { Type } from "class-transformer";
+import { EntryStatus } from "@prisma/client";
 
 /**
  * Query parameters for entry-centered graph view
@@ -12,3 +13,24 @@ export class GraphQueryDto {
   @Max(5)
   depth?: number = 1;
 }
+
+/**
+ * Query parameters for full graph view with filtering
+ */
+export class GraphFilterDto {
+  @IsOptional()
+  @IsArray()
+  @IsString({ each: true })
+  tags?: string[];
+
+  @IsOptional()
+  @IsEnum(EntryStatus)
+  status?: EntryStatus;
+
+  @IsOptional()
+  @Type(() => Number)
+  @IsInt()
+  @Min(1)
+  @Max(1000)
+  limit?: number;
+}

apps/api/src/knowledge/dto/index.ts

@@ -5,6 +5,6 @@ export { CreateTagDto } from "./create-tag.dto";
 export { UpdateTagDto } from "./update-tag.dto";
 export { RestoreVersionDto } from "./restore-version.dto";
 export { SearchQueryDto, TagSearchDto, RecentEntriesDto } from "./search-query.dto";
-export { GraphQueryDto } from "./graph-query.dto";
+export { GraphQueryDto, GraphFilterDto } from "./graph-query.dto";
 export { ExportQueryDto, ExportFormat } from "./import-export.dto";
 export type { ImportResult, ImportResponseDto } from "./import-export.dto";