feat: implement database transaction management for critical operations

[RUKAYAT-CODER]

Summary

Implements explicit database transaction management for critical multi-write operations in the meridian-api, addressing the risk of partially persisted data when one write succeeds and a subsequent one fails.

Changes

• src/common/providers/database-transaction.provider.ts — New reusable DatabaseTransactionProvider service wrapping any callback in a QueryRunner transaction: connects, begins, commits on success, rolls back + logs on failure, and always releases the runner.

• src/post/provider/post.service.ts — createPost and UpdatePost now execute inside explicit QueryRunner transactions. The post row, cascaded MetaOption save, and tag join-table writes are applied atomically; any failure triggers a full rollback.

• src/common/providers/database-transaction.provider.spec.ts — Unit tests covering commit on success and rollback-on-error for the new provider.

• src/post/provider/post.service.spec.ts — Updated tests covering both happy-path transaction commits and rollback scenarios for createPost and UpdatePost.

closes #524

[Qoder-Undefined]

LGTM