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/knowledge.module.ts

@@ -4,13 +4,26 @@ import { AuthModule } from "../auth/auth.module";
 import { KnowledgeService } from "./knowledge.service";
 import { KnowledgeController } from "./knowledge.controller";
 import { SearchController } from "./search.controller";
-import { LinkResolutionService } from "./services/link-resolution.service";
-import { SearchService } from "./services/search.service";
+import { KnowledgeStatsController } from "./stats.controller";
+import {
+  LinkResolutionService,
+  SearchService,
+  LinkSyncService,
+  GraphService,
+  StatsService,
+} from "./services";
 
 @Module({
   imports: [PrismaModule, AuthModule],
-  controllers: [KnowledgeController, SearchController],
-  providers: [KnowledgeService, LinkResolutionService, SearchService],
+  controllers: [KnowledgeController, SearchController, KnowledgeStatsController],
+  providers: [
+    KnowledgeService,
+    LinkResolutionService,
+    SearchService,
+    LinkSyncService,
+    GraphService,
+    StatsService,
+  ],
   exports: [KnowledgeService, LinkResolutionService, SearchService],
 })
 export class KnowledgeModule {}