Add Referral Factory startup offer

[offer-catalog-worker]

What changed

Adds Referral Factory and its startup-specific offer: 50% off a qualifying subscription.

Closes #801

Sources

• https://referral-factory.com/startup-program

Certification

• I changed only allowed repository content and did not mix Entity YAML with other paths.
• Public sources substantiate every submitted factual value; any Sourcey-written prose is a neutral summary, not a fabricated vendor quote.
• I did not author verification, provenance, freshness, or signature claims.
• My commits include a Signed-off-by line.