feat: add backlinks display and wiki-link rendering (closes #62, #64)

Implements two key knowledge module features:

**#62 - Backlinks Display:**
- Added BacklinksList component to show entries that link to current entry
- Fetches backlinks from /api/knowledge/entries/:slug/backlinks
- Displays entry title, summary, and link context
- Clickable links to navigate to linking entries
- Loading, error, and empty states

**#64 - Wiki-Link Rendering:**
- Added WikiLinkRenderer component to parse and render wiki-links
- Supports [[slug]] and [[slug|display text]] syntax
- Converts wiki-links to clickable navigation links
- Distinct styling (blue color, dotted underline)
- XSS protection via HTML escaping
- Memoized HTML processing for performance

**Components:**
- BacklinksList.tsx - Backlinks display with empty/loading/error states
- WikiLinkRenderer.tsx - Wiki-link parser and renderer
- Updated EntryViewer.tsx to use WikiLinkRenderer
- Integrated BacklinksList into entry detail page

**API:**
- Added fetchBacklinks() function in knowledge.ts
- Added KnowledgeBacklink type to shared types

**Tests:**
- Comprehensive tests for BacklinksList (8 tests)
- Comprehensive tests for WikiLinkRenderer (14 tests)
- All tests passing with Vitest

**Type Safety:**
- Strict TypeScript compliance
- No 'any' types
- Proper error handling

apps/web/src/lib/api/knowledge.ts

@@ -181,6 +181,55 @@ export async function restoreVersion(
   );
 }
 
+/**
+ * Fetch backlinks for an entry (entries that link to this entry)
+ */
+export async function fetchBacklinks(slug: string): Promise<{
+  entry: { id: string; slug: string; title: string };
+  backlinks: Array<{
+    id: string;
+    sourceId: string;
+    targetId: string;
+    linkText: string;
+    displayText: string;
+    positionStart: number;
+    positionEnd: number;
+    resolved: boolean;
+    context: string | null;
+    createdAt: Date;
+    source: {
+      id: string;
+      title: string;
+      slug: string;
+      summary?: string | null;
+    };
+  }>;
+  count: number;
+}> {
+  return apiGet<{
+    entry: { id: string; slug: string; title: string };
+    backlinks: Array<{
+      id: string;
+      sourceId: string;
+      targetId: string;
+      linkText: string;
+      displayText: string;
+      positionStart: number;
+      positionEnd: number;
+      resolved: boolean;
+      context: string | null;
+      createdAt: Date;
+      source: {
+        id: string;
+        title: string;
+        slug: string;
+        summary?: string | null;
+      };
+    }>;
+    count: number;
+  }>(`/api/knowledge/entries/${slug}/backlinks`);
+}
+
 /**
  * Mock entries for development (until backend endpoints are ready)
  */