Add Supabase RPC integration tests for session usage accounting

[Taleef7]

Context

Unit tests cover scheduling and status-transition math, but the DB-backed RPC behavior still needs local Supabase verification. This is the accounting core: done and no_show_student consume a session, no_show_tutor and rescheduled do not.

Acceptance criteria

• Add a local Supabase-backed test or documented verification harness for increment_sessions_used, decrement_sessions_used, and tutor_update_session.
• Cover transitions between scheduled, done, no_show_student, no_show_tutor, and rescheduled without double-incrementing.
• Confirm RPC permissions only allow intended callers/service role paths.
• Ensure tests can be run from a clean npx supabase db reset state.
• Document the command and expected output.

References

• docs/plan-CorvEd.md section 4.5 testing
• docs/MVP.md session/no-show policy
• lib/services/__tests__/session-status-transitions.test.ts
• supabase/migrations/*increment_sessions_used*