docs: fix broken SAT quick-start example in README

[dmchaledev]

Summary

The SAT "Quick Start" curl example in README.md was inconsistent with sat/openapi.yaml, so copy-pasting it would fail:

• It targeted https://api.hailbytes.com/sat/v1/users, but SAT's spec (sat/openapi.yaml:31-33) declares a relative /api server — SAT is self-hosted and has no fixed public host, unlike ASM.
• It POSTed to /users, but the spec's actual POST /users/ operation (sat/openapi.yaml:1184-1206) creates an admin/system user (requires modify_system permission) and has no requestBody schema defined, so the example's {"email": ..., "group_id": ...} payload was never documented anywhere.
• The spec explicitly notes (sat/openapi.yaml:14-17) the API is unversioned under /api/, with a versioned alias at /api/v1/ — the README used neither shape consistently.

Fix

Replaced the example with the actual, documented way to enroll users in SAT: POST /groups/ (sat/openapi.yaml:899-917), whose request body matches the Group/Target schemas (sat/openapi.yaml:305-332) — a group with a targets array of {email, first_name, last_name, ...}. Also clarified that SAT is self-hosted, so the host is instance-specific rather than a fixed api.hailbytes.com domain.

Test plan

• Verified POST /groups/ and its Group/Target schemas exist in sat/openapi.yaml and match the new example's payload shape
• Confirmed the ASM example directly above was unaffected (it already matches asm/openapi.yaml's https://api.hailbytes.com/asm/v1 server)
• No code/CI changes required; this is a docs-only fix

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