feat(05-02): add SearchProvider backend with model, CRUD, and defaults

- Prisma model SearchProvider with userId/tenantId scoping
- Three default providers (Google/Bing/DuckDuckGo) as constants, always returned without DB seed
- GET/POST/DELETE search-providers endpoints on DashboardController
- Ownership verification on delete (T-05-07), default providers cannot be deleted
- CreateSearchProviderDto with class-validator: urlTemplate must contain {query} (T-05-08)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/src/dashboard/dto/create-search-provider.dto.ts

@@ -0,0 +1,18 @@
+import { IsNotEmpty, IsString, Matches } from 'class-validator';
+
+/**
+ * DTO for creating a custom search provider (D-15).
+ * urlTemplate must contain {query} placeholder for the search term.
+ */
+export class CreateSearchProviderDto {
+  @IsString()
+  @IsNotEmpty()
+  name!: string;
+
+  @IsString()
+  @IsNotEmpty()
+  @Matches(/\{query\}/, {
+    message: 'urlTemplate must contain {query}',
+  })
+  urlTemplate!: string;
+}