feat: add categories and favorites schemas with integrations

Added `categories` and `favorites` database schemas with full type inference support. Integrated categories into `content` schema with new properties (`categoryId`, `slug`, `views`, and `usageCount`). Updated `tags` schema to include `userId` with reference to `users`. Exported new schemas in index for broader usage.

backend/src/database/schemas/index.ts

@@ -1,6 +1,8 @@
 export * from "./api_keys";
 export * from "./audit_logs";
+export * from "./categories";
 export * from "./content";
+export * from "./favorites";
 export * from "./rbac";
 export * from "./reports";
 export * from "./sessions";