[M2][Epic 2.2] Add API polling and file upload source types

[POWDER-RANGER]

Problem

Extend POST /api/sources to support API polling (simple GET requests) and CSV/JSON uploads.

Tasks

• Add source type validation (api|rss|upload)
• Implement API source: store endpoint URL, poll interval
• Implement API source: HTTP client with error handling
• Implement upload source: accept CSV/JSON via multipart form
• Add data normalization layer (CSV → documents, JSON → documents)
• Write integration tests for each source type
• Update frontend to support all 3 types

Acceptance Criteria

• ✅ POST /api/sources accepts RSS + API + Upload
• ✅ Data normalized correctly (schema validated)
• ✅ Ingestion works identically across all source types

Priority: P1 - High
Labels: backend, ingestion, M2, P1