fix: migration 009 - change profile_id from VARCHAR(36) to UUID

Profile IDs are UUID type in the profiles table, not VARCHAR.
This was causing foreign key constraint error on migration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/migrations/009_sleep_log.sql

@@ -4,7 +4,7 @@
 
 CREATE TABLE IF NOT EXISTS sleep_log (
     id SERIAL PRIMARY KEY,
-    profile_id VARCHAR(36) NOT NULL REFERENCES profiles(id) ON DELETE CASCADE,
+    profile_id UUID NOT NULL REFERENCES profiles(id) ON DELETE CASCADE,
     date DATE NOT NULL,
     bedtime TIME,
     wake_time TIME,