fix: remove account-enumeration oracle in match_users RPC (#1927)

[atul-upadhyay-7]

Fixes #1927

Problem

match_users is a SECURITY DEFINER function GRANTed to authenticated, and its only notion of "self" is a caller-supplied target_email argument (WHERE p.email != target_email). Any logged-in user could invoke it directly with an arbitrary email and use the returned profile set as an account-existence oracle, plus dump the user directory (names, skills, interests, teach/learn subjects) without using the app.

Changes

New migration 20260805000004_secure_match_users_rpc.sql:

• REVOKE ALL ... FROM PUBLIC and drop the authenticated grant — match_users is now callable only by the service role (the backend /api/match/recommendations endpoint, which passes the caller's own email). service_role needs no explicit grant for SECURITY DEFINER functions.
• Defense in depth inside the function:
  • require a non-empty target_email (otherwise the query silently returns the whole directory),
  • when auth.uid() is present (i.e. an authenticated caller somehow reaches it), require target_email to equal the caller's own email from auth.users, else RAISE EXCEPTION,
  • exclude the caller's own profile case-insensitively (lower(p.email) <> lower(target_email)).

Verification

• The backend (backend/controllers/matchController.js) calls the RPC via the service-role client with req.user.email; auth.uid() is NULL on that path, so the new guard does not affect it.
• The frontend never calls match_users directly (only the backend endpoint).
• Non-admin admin_get_all_profiles remains properly gated — unchanged.

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Reviewing files that changed from the base of the PR and between 880eed9 and ac3369e.

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• supabase/migrations/20260805000004_secure_match_users_rpc.sql

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