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/entities/graph.entity.ts

@@ -0,0 +1,40 @@
+/**
+ * Represents a node in the knowledge graph
+ */
+export interface GraphNode {
+  id: string;
+  slug: string;
+  title: string;
+  summary: string | null;
+  tags: Array<{
+    id: string;
+    name: string;
+    slug: string;
+    color: string | null;
+  }>;
+  depth: number;
+}
+
+/**
+ * Represents an edge/link in the knowledge graph
+ */
+export interface GraphEdge {
+  id: string;
+  sourceId: string;
+  targetId: string;
+  linkText: string;
+}
+
+/**
+ * Entry-centered graph response
+ */
+export interface EntryGraphResponse {
+  centerNode: GraphNode;
+  nodes: GraphNode[];
+  edges: GraphEdge[];
+  stats: {
+    totalNodes: number;
+    totalEdges: number;
+    maxDepth: number;
+  };
+}