Add Twelve Data Early Stage Program

[huangm199]

Closes #689

Summary

• adds Twelve Data as one new vendor YAML
• adds the official Early Stage Program with exactly one offer: 20% off plans for 12 months
• models the published funding, company-age, team-size, and legal-entity/company-domain eligibility
• cites only the first-party Twelve Data startup page

Verification

• pinned Sourcey Catalog Verifier: {"status":"valid","vendors":1,"programs":1,"offers":1}
• DCO sign-off included
• data-only change: one new file under vendors/tw/

First-party source: https://twelvedata.com/startups

Prepared for Frantic bounty #120.