Drop the UNIQUE constraint on sales_agents.email (migration 084) so a single
agent (person/company) can hold several referral codes, each with its own
client discount and commission split. All commission lookups already key on
the unique agent_code, so no lookup logic changes.
Agent-creation endpoint now:
- accepts repeat emails (creates an additional code instead of 409)
- accepts client_discount_value, commission_type, commission_pct per code
- reports existing codes for the email in the response
Both Jay Kordic codes (REF-JKORDIC 7%/12%, REF-JAYK05 5%/15%) now share his
real email jay_kordic@thehorizongroup.biz.
