[Feature]: Integrate manual uploads into the unified ingestion artifact store

[VladyslavHandzha]

Summary

Store manually uploaded files as unified Artifact entities so they use the same ingestion, retrieval, AI indexing, and deletion flow as artifacts from other sources.

Acceptance Criteria

• Persist uploaded files as project-scoped Artifact entities.
• Track upload success and failure through ingestion runs.
• Send uploaded artifacts to the AI service for indexing.
• Delete the corresponding artifact when an upload is deleted.
• Send deleted artifact IDs to the AI service for deindexing.
• Add tests for persistence, deletion, failure tracking, and AI synchronization.