feat(#66): implement tag filtering in search API endpoint

Add support for filtering search results by tags in the main search endpoint.

Changes:
- Add tags parameter to SearchQueryDto (comma-separated tag slugs)
- Implement tag filtering in SearchService.search() method
- Update SQL query to join with knowledge_entry_tags when tags provided
- Entries must have ALL specified tags (AND logic)
- Add tests for tag filtering (2 controller tests, 2 service tests)
- Update endpoint documentation
- Fix non-null assertion linting error

The search endpoint now supports:
- Full-text search with ranking (ts_rank)
- Snippet generation with highlighting (ts_headline)
- Status filtering
- Tag filtering (new)
- Pagination

Example: GET /api/knowledge/search?q=api&tags=documentation,tutorial

All tests pass (25 total), type checking passes, linting passes.

Fixes #66

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/api/src/knowledge/search.controller.spec.ts

@@ -55,15 +55,11 @@ describe("SearchController", () => {
         limit: 20,
       });
 
-      expect(mockSearchService.search).toHaveBeenCalledWith(
-        "test",
-        mockWorkspaceId,
-        {
-          status: undefined,
-          page: 1,
-          limit: 20,
-        }
-      );
+      expect(mockSearchService.search).toHaveBeenCalledWith("test", mockWorkspaceId, {
+        status: undefined,
+        page: 1,
+        limit: 20,
+      });
       expect(result).toEqual(mockResult);
     });
 
@@ -79,15 +75,54 @@ describe("SearchController", () => {
         status: EntryStatus.PUBLISHED,
       });
 
-      expect(mockSearchService.search).toHaveBeenCalledWith(
-        "test",
-        mockWorkspaceId,
-        {
-          status: EntryStatus.PUBLISHED,
-          page: undefined,
-          limit: undefined,
-        }
-      );
+      expect(mockSearchService.search).toHaveBeenCalledWith("test", mockWorkspaceId, {
+        status: EntryStatus.PUBLISHED,
+        page: undefined,
+        limit: undefined,
+      });
+    });
+
+    it("should pass tags filter to service", async () => {
+      mockSearchService.search.mockResolvedValue({
+        data: [],
+        pagination: { page: 1, limit: 20, total: 0, totalPages: 0 },
+        query: "test",
+      });
+
+      await controller.search(mockWorkspaceId, {
+        q: "test",
+        tags: ["api", "documentation"],
+      });
+
+      expect(mockSearchService.search).toHaveBeenCalledWith("test", mockWorkspaceId, {
+        status: undefined,
+        page: undefined,
+        limit: undefined,
+        tags: ["api", "documentation"],
+      });
+    });
+
+    it("should pass both status and tags filters to service", async () => {
+      mockSearchService.search.mockResolvedValue({
+        data: [],
+        pagination: { page: 1, limit: 20, total: 0, totalPages: 0 },
+        query: "test",
+      });
+
+      await controller.search(mockWorkspaceId, {
+        q: "test",
+        status: EntryStatus.PUBLISHED,
+        tags: ["api"],
+        page: 2,
+        limit: 10,
+      });
+
+      expect(mockSearchService.search).toHaveBeenCalledWith("test", mockWorkspaceId, {
+        status: EntryStatus.PUBLISHED,
+        page: 2,
+        limit: 10,
+        tags: ["api"],
+      });
     });
   });
 
@@ -128,15 +163,11 @@ describe("SearchController", () => {
         status: EntryStatus.DRAFT,
       });
 
-      expect(mockSearchService.searchByTags).toHaveBeenCalledWith(
-        ["api"],
-        mockWorkspaceId,
-        {
-          status: EntryStatus.DRAFT,
-          page: undefined,
-          limit: undefined,
-        }
-      );
+      expect(mockSearchService.searchByTags).toHaveBeenCalledWith(["api"], mockWorkspaceId, {
+        status: EntryStatus.DRAFT,
+        page: undefined,
+        limit: undefined,
+      });
     });
   });
 
@@ -156,11 +187,7 @@ describe("SearchController", () => {
         limit: 10,
       });
 
-      expect(mockSearchService.recentEntries).toHaveBeenCalledWith(
-        mockWorkspaceId,
-        10,
-        undefined
-      );
+      expect(mockSearchService.recentEntries).toHaveBeenCalledWith(mockWorkspaceId, 10, undefined);
       expect(result).toEqual({
         data: mockEntries,
         count: 1,
@@ -172,11 +199,7 @@ describe("SearchController", () => {
 
       await controller.recentEntries(mockWorkspaceId, {});
 
-      expect(mockSearchService.recentEntries).toHaveBeenCalledWith(
-        mockWorkspaceId,
-        10,
-        undefined
-      );
+      expect(mockSearchService.recentEntries).toHaveBeenCalledWith(mockWorkspaceId, 10, undefined);
     });
 
     it("should pass status filter to service", async () => {