Allow multiple referral codes per sales agent

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.

api/migrations/084_multi_code_per_agent.sql

@@ -0,0 +1,23 @@
+-- 084_multi_code_per_agent.sql
+-- Allow a single sales agent (person/company) to have MULTIPLE referral codes,
+-- each with its own client discount and agent commission split.
+--
+-- Background: each sales_agents row already represents one referral code
+-- (it owns a unique agent_code and links to one discount_code). The only thing
+-- that blocked the same person from having several codes was the UNIQUE
+-- constraint on sales_agents.email. We drop that constraint (keeping a plain
+-- index for lookups) so the same email can back several codes/splits.
+--
+-- All commission/referral lookups key on the unique agent_code, so no other
+-- logic changes are required at the DB level.
+
+BEGIN;
+
+-- Drop the UNIQUE constraint on email (name is auto-generated by Postgres).
+ALTER TABLE sales_agents DROP CONSTRAINT IF EXISTS sales_agents_email_key;
+
+-- Keep a non-unique index so email lookups stay fast.
+-- (idx_agents_email already exists from migration 011; create defensively.)
+CREATE INDEX IF NOT EXISTS idx_agents_email ON sales_agents (email);
+
+COMMIT;