chore(schema): allow string-valued issue/volume in citation references (#37)

Some Gems & Gemology entries cite combined issues (e.g. "1–4") which can't
be expressed as integers. Widen the journal-reference Zod schema to accept
either int or string for volume and issue.

Unblocks the v1.3.0 content sync (gemmology-knowledge equipment/conoscope.yaml
declares issue: "1–4" against Read 2014 chapter range).

Author: Bissbert