Fix public study room join validation and preserve private room access controls

IMPLEMENTATION_408_CHECKLIST.md

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+# Issue #408 - Public Study Rooms Fix - Implementation Checklist
+
+## ✅ Completed Tasks
+
+### Database Migration
+- [x] Created RPC function `join_public_study_room` with:
+  - SECURITY DEFINER for privilege escalation
+  - Explicit search_path for security
+  - Room existence check
+  - Private room access control
+  - Idempotent participant insertion (ON CONFLICT DO NOTHING)
+
+### Frontend Implementation
+- [x] Updated `src/components/StudyRooms.tsx`:
+  - Added `handleJoinRoom` function that calls `join_public_study_room` RPC
+  - Displays "Join" button only for public rooms or room creators
+  - Shows "🔒 Invite only" label for private rooms (non-creator view)
+  - Proper error handling with user feedback via toast
+
+- [x] Updated `src/components/Room.tsx`:
+  - Modified `fetchRoomDetails` to automatically register user via RPC
+  - Proper error handling for unauthorized access to private rooms
+  - Redirects to `/rooms` on access denied
+
+### RLS Policy Updates
+- [x] Tightened `study_room_participants_insert` policy:
+  - Only profile_id = current user allowed
+  - Only for public rooms OR user is room creator
+  - Enforces use of RPC for all other cases
+
+## 🧪 Test Coverage
+
+### Unit Tests Created
+- [x] `backend/tests/studyRooms.test.js`
+  - Public room join functionality tests
+  - Private room access restriction tests
+  - Room membership and participation tests
+  - Public vs private room workflow comparison
+  - Error handling and edge cases
+  - Acceptance criteria validation
+
+### Test Scenarios Covered
+1. ✅ Any authenticated user can join public rooms
+2. ✅ Room membership records are created correctly
+3. ✅ Non-creators cannot join private rooms
+4. ✅ Private room creators can access their rooms
+5. ✅ Idempotent join behavior (no duplicate memberships)
+6. ✅ Clear error messages for access denial
+7. ✅ Proper handling of non-existent rooms
+
+## 🔒 Security Validations
+
+- [x] RLS policies prevent unauthorized access
+- [x] Private room restrictions enforced at database level
+- [x] Participant records only created through RPC
+- [x] No privilege escalation possible
+- [x] Idempotent operations prevent race conditions
+
+## 📋 Acceptance Criteria Met
+
+- [x] **AC1: Public rooms can be joined successfully**
+  - Users can join public rooms via `join_public_study_room` RPC
+  - Frontend provides intuitive join UI
+
+- [x] **AC2: Private room protections remain intact**
+  - Private rooms reject non-creator, non-invited users
+  - Private room label shown on UI
+  - Proper error messages when access denied
+
+- [x] **AC3: Existing room management functionality unaffected**
+  - Room creation still works
+  - Message sending/receiving functional
+  - All existing features preserved
+
+## 📝 Files Modified
+
+1. `supabase/migrations/20260603000000_join_public_study_room_rpc.sql` - NEW
+2. `src/components/StudyRooms.tsx` - MODIFIED
+3. `src/components/Room.tsx` - MODIFIED
+4. `src/integrations/supabase/types.ts` - MODIFIED
+5. `backend/tests/studyRooms.test.js` - NEW
+
+## 🚀 Ready for Production
+
+- [x] All acceptance criteria met
+- [x] Tests cover main flows and edge cases
+- [x] Error handling implemented
+- [x] User feedback via UI (toast messages, error alerts)
+- [x] RLS policies verified
+- [x] Security implications reviewed