feat: add knowledge graph views and stats (closes #73, closes #74)

Issue #73 - Entry-Centered Graph View:
- Added GET /api/knowledge/entries/:id/graph endpoint with depth parameter
- Returns entry + connected nodes with link relationships
- Created GraphService for graph traversal using BFS
- Added EntryGraphViewer component for frontend
- Integrated graph view tab into entry detail page

Issue #74 - Graph Statistics Dashboard:
- Added GET /api/knowledge/stats endpoint
- Returns overview stats (entries, tags, links by status)
- Includes most connected entries, recent activity, tag distribution
- Created StatsDashboard component with visual stats
- Added route at /knowledge/stats

Backend:
- GraphService: BFS-based graph traversal with configurable depth
- StatsService: Parallel queries for comprehensive statistics
- GraphQueryDto: Validation for depth parameter (1-5)
- Entity types for graph nodes/edges and statistics
- Unit tests for both services

Frontend:
- EntryGraphViewer: Entry-centered graph visualization
- StatsDashboard: Statistics overview with charts
- Graph view tab on entry detail page
- API client functions for new endpoints
- TypeScript strict typing throughout

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

@@ -0,0 +1,14 @@
+import { IsOptional, IsInt, Min, Max } from "class-validator";
+import { Type } from "class-transformer";
+
+/**
+ * Query parameters for entry-centered graph view
+ */
+export class GraphQueryDto {
+  @IsOptional()
+  @Type(() => Number)
+  @IsInt()
+  @Min(1)
+  @Max(5)
+  depth?: number = 1;
+}

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

@@ -8,3 +8,4 @@ export {
   TagSearchDto,
   RecentEntriesDto,
 } from "./search-query.dto";
+export { GraphQueryDto } from "./graph-query.dto";