* ⚡ Optimize demo user setup for performance tests
Use UUID-based demo emails to prevent concurrent profile collisions.\nUse fast PBKDF2 hashing for demo profiles while keeping regular user hashing unchanged.\nAdd focused coverage for hashing, email uniqueness, and the feature flag.\n\nAI-assisted-by: gpt-5.6-luna
* 🐛 Harden font upload test setup
Report upload-session errors before chunk validation.
Skip chunk uploads when the session ID is invalid.
Remove unnecessary Mockery state from the foreign-font test.
AI-assisted-by: gpt-5.6-luna
* ✨ Add demo profile purge task
Schedule delayed deletion for demo profiles through the worker system.
Restore normal profile filtering and cover the purge handler with tests.
AI-assisted-by: gpt-5.6-luna
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