Feature Request: Multi-URL Scraping with Domain Grouping & RAG-Ready Export

Closes #39
### Problem
Currently, SCRAG processes URLs individually and does not provide:
- Multi-URL concurrent scraping
- Grouping of results by domain
- Built-in rate limiting per domain
- RAG-ready exports (chunked JSON suitable for LLM ingestion)

This limits its usability for real-world RAG pipelines where users often scrape multiple sources simultaneously.

### Proposed Solution
Introduce a backend enhancement that supports:
- Concurrent scraping of multiple URLs
- Per-domain rate limiting (e.g., 1 request/sec)
- Grouping results by domain
- Pass/fail status tracking per URL
- Content deduplication using hashing
- Export options:
- CSV for visibility/debugging
- Chunked JSON optimized for RAG pipelines

### Benefits
- Improves scalability and robustness
- Makes SCRAG immediately usable in RAG workflows
- Aligns with real-world scraping + LLM ingestion needs

### Additional Notes
This proposal is orthogonal to previous discussions around async HTTP extractors (e.g., aiohttp). It focuses on higher-level orchestration, domain-level controls, and RAG-oriented outputs, and can leverage existing async extractors internally.

The issue names no files, tests, or entry points, so start by locating the existing scraping orchestration and async extractor interfaces. Done would require implementing and validating concurrent multi-URL scraping, per-domain rate limiting, grouping, status tracking, deduplication, and CSV and chunked JSON exports.

Written by the indexing model from the issue text.