Add Directory of Open Access Journal (DOAJ) as new source

## Problem Statement
The quantifying commons project currently lacks coverage of open access academic journals, which represent a significant portion of the scholarly commons. DOAJ provides comprehensive metadata on over 22,000 open access journals with detailed Creative Commons licensing information. DOAJ provides rich metadata including `license types`, `publisher information`, `country codes`, `subject classifications`, and `open access start dates`, making it ideal for detailed commons analysis.

### Relevant API Links
- **Api Version**: v4
- **Base URL**: https://doaj.org/api/
- **Journals Endpoint**: https://doaj.org/api/v4/search/journals/
- **Articles Endpoint**: https://doaj.org/api/v4/search/articles/
- **API Documentation**: https://doaj.org/api/docs

### Useful Search Fields
- **License Information**: `bibjson.license[].type`
- **Publisher Details**: `bibjson.publisher.name`, `bibjson.publisher.country`
- **Open Access Start**: `bibjson.oa_start`
- **Subject Classification**: `bibjson.subject[].code`, `bibjson.subject[].term`
- **Language**: `bibjson.language[]`
- **Last Updated**: `last_updated`
- **Created Date**: `created_date`

### API Limitations
- **Rate Limits**: Current delays 0.5 seconds between requests
- **Articles**: No direct license information available
- **Pagination**: Maximum `pageSize` appears to be 100
- **Historical Data**: License information may reflect current status, not original licensing
- **Date Filtering**: Only available through `oa_start` field, not license adoption date
- **Date Back Default**: 2002 (to avoid CC license false positives)

## Additional context

The DOAJ data source provides journal-level licensing policies rather than individual article licenses. The Creative Commons license information collected represents the default licensing framework that journals have committed to implementing, as declared in their editorial policies and reported to DOAJ **(https://doaj.org/apply/guide/)**.

This data does not reflect the actual licensing status of individual published articles, which may vary from the journal's stated policy due to author choices, publisher agreements, or historical publishing practices predating the journal's current licensing framework.

The quantification derived from DOAJ therefore represents the scope of journals committed to Creative Commons licensing rather than a direct count of CC-licensed articles. This distinction is particularly relevant for temporal analysis, as journals may have adopted CC licensing policies after publishing content under different terms, and individual authors within CC-committed journals may still select more restrictive licensing options where permitted by editorial policy.

The DOAJ API documentation **(https://doaj.org/api/docs)** confirms that article-level license metadata is not available through their search endpoints.

Users of this data should interpret DOAJ-derived metrics as indicators of institutional commitment to open access licensing within the scholarly publishing ecosystem, rather than as precise measurements of the volume of CC-licensed scholarly content. This limitation does not diminish the value of DOAJ data for understanding trends in open access policy adoption, geographic distribution of OA-committed publishers, and the evolution of scholarly publishing practices toward more open licensing frameworks. Additional context on DOAJ's selection criteria and quality standards can be found in their information for publishers **(https://doaj.org/publishers/)** and their principles of transparency and best practice **(https://doaj.org/bestpractice/).**

## Implementation

- [x] I would be interested in implementing this feature.