Add support for flagging legacy DOI citation styles in Reference classification

[Aakash-pal]

Summary

While DOI extraction is supported, the system does not currently distinguish between modern and legacy DOI citation formats.

Adding this distinction at the Reference level could improve downstream analysis and reporting.

Problem

DOIs may appear in different citation styles, for example:

Modern:
https://doi.org/10.xxxx/...
Legacy:
DOI: 10.xxxx/...

Both are valid and can be extracted, but there is currently no way to identify whether a DOI originated from a legacy citation format.

Proposed Enhancement

Extend Reference (in backends.py) to:

Detect when a DOI originates from a legacy citation pattern
Store this as additional metadata (e.g., legacy_format=True)
Example (conceptual)
Input: "DOI: 10.1007/s11089-018-0833-1"
Reference(
ref="10.1007/s11089-018-0833-1",
reftype="doi",
legacy_format=True
)

Expected Behavior

DOI extraction remains unchanged
Additional metadata is available for distinguishing citation styles

Notes

This is a feature enhancement, not a bug fix
Should not interfere with existing extraction logic
Can be extended later for reporting/UI use cases in downstream projects (e.g., rottingresearch)"

[Aakash-pal]

Hi @marshalmiller,

You can assign this to me once you review issue #470

Thanks,
Aakash Pal.