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/prisma/schema.prisma

@@ -134,3 +134,15 @@ model WidgetInstance {
   @@index([userId])
   @@index([tenantId])
 }
+
+model SearchProvider {
+  id          String   @id @default(uuid())
+  userId      String?
+  tenantId    String?
+  name        String
+  urlTemplate String
+  isDefault   Boolean  @default(false)
+  createdAt   DateTime @default(now())
+
+  @@index([userId])
+}