RLS Allows SELECT But Blocks INSERT/UPDATE

1. INSERT policy doesn't exist or uses wrong auth check
2. WITH CHECK clause missing from INSERT policy
3. UPDATE policy missing or references wrong columns
4. Policy targets wrong role (anon vs authenticated)
5. Multiple conflicting policies with AND logic

Create separate policies for each operation:

-- SELECT policy
CREATE POLICY "Read own data" ON users
FOR SELECT USING (auth.uid() = id);

-- INSERT policy  
CREATE POLICY "Create own user" ON users
FOR INSERT WITH CHECK (auth.uid() = id);

-- UPDATE policy
CREATE POLICY "Update own data" ON users
FOR UPDATE USING (auth.uid() = id)
WITH CHECK (auth.uid() = id);