Initial commit: PastPaper Master full stack

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

supabase/migrations/004_decouple_course_library_from_auth.sql

@@ -0,0 +1,30 @@
+-- ============================================
+-- PastPaper Master — Decouple course library papers from auth users
+-- Version: 004
+-- Date: 2026-03-24
+-- ============================================
+
+-- Course-library papers should not depend on a concrete auth.users row.
+-- User-uploaded papers still keep user_id populated.
+ALTER TABLE papers
+  ALTER COLUMN user_id DROP NOT NULL;
+
+-- Keep existing FK so user-owned papers can still reference auth.users,
+-- while course-library rows simply use NULL.
+
+-- Tighten the intended invariant with a check constraint:
+-- - user_upload rows must have user_id
+-- - course_library rows must not have user_id
+ALTER TABLE papers
+  DROP CONSTRAINT IF EXISTS papers_source_kind_user_id_check;
+
+ALTER TABLE papers
+  ADD CONSTRAINT papers_source_kind_user_id_check
+  CHECK (
+    (source_kind = 'user_upload' AND user_id IS NOT NULL)
+    OR
+    (source_kind = 'course_library' AND user_id IS NULL)
+  );
+
+-- Existing RLS policies continue to apply to user-owned rows.
+-- Course-library rows are accessed through the backend service role.