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/web/src/app/(authenticated)/knowledge/[slug]/page.tsx

@@ -7,6 +7,7 @@ import { EntryStatus, Visibility } from "@mosaic/shared";
 import { EntryViewer } from "@/components/knowledge/EntryViewer";
 import { EntryEditor } from "@/components/knowledge/EntryEditor";
 import { EntryMetadata } from "@/components/knowledge/EntryMetadata";
+import { EntryGraphViewer } from "@/components/knowledge/EntryGraphViewer";
 import { fetchEntry, updateEntry, deleteEntry, fetchTags } from "@/lib/api/knowledge";
 
 /**
@@ -20,6 +21,7 @@ export default function EntryPage() {
 
   const [entry, setEntry] = useState<KnowledgeEntryWithTags | null>(null);
   const [isEditing, setIsEditing] = useState(false);
+  const [showGraph, setShowGraph] = useState(false);
   const [isLoading, setIsLoading] = useState(true);
   const [isSaving, setIsSaving] = useState(false);
   const [error, setError] = useState<string | null>(null);
@@ -268,10 +270,42 @@ export default function EntryPage() {
         </div>
       )}
 
+      {/* View Tabs */}
+      {!isEditing && (
+        <div className="mb-4 border-b border-gray-200 dark:border-gray-700">
+          <div className="flex gap-4">
+            <button
+              onClick={() => setShowGraph(false)}
+              className={`px-4 py-2 text-sm font-medium border-b-2 transition-colors ${
+                !showGraph
+                  ? "border-blue-500 text-blue-600 dark:text-blue-400"
+                  : "border-transparent text-gray-500 dark:text-gray-400 hover:text-gray-700 dark:hover:text-gray-300"
+              }`}
+            >
+              Content
+            </button>
+            <button
+              onClick={() => setShowGraph(true)}
+              className={`px-4 py-2 text-sm font-medium border-b-2 transition-colors ${
+                showGraph
+                  ? "border-blue-500 text-blue-600 dark:text-blue-400"
+                  : "border-transparent text-gray-500 dark:text-gray-400 hover:text-gray-700 dark:hover:text-gray-300"
+              }`}
+            >
+              Graph View
+            </button>
+          </div>
+        </div>
+      )}
+
       {/* Content */}
       <div className="mb-6">
         {isEditing ? (
           <EntryEditor content={editContent} onChange={setEditContent} />
+        ) : showGraph ? (
+          <div className="bg-white dark:bg-gray-800 rounded-lg shadow-sm border border-gray-200 dark:border-gray-700 overflow-hidden" style={{ height: '600px' }}>
+            <EntryGraphViewer slug={slug} initialDepth={1} />
+          </div>
         ) : (
           <EntryViewer entry={entry} />
         )}

apps/web/src/app/(authenticated)/knowledge/stats/page.tsx

@@ -0,0 +1,5 @@
+import { StatsDashboard } from "@/components/knowledge";
+
+export default function KnowledgeStatsPage() {
+  return <StatsDashboard />;
+}