Critical fixes: - Remove duplicate DeleteAccountHandler in tests, use actual production handler - Add comprehensive integration test for complete deletion flow - Tests now use middleware.SetTestUserDID() instead of X-User-DID header Error handling improvements: - Add InvalidDIDError domain type for DID validation errors - Handler returns specific errors for timeout (504) and cancellation (400) - Marshal JSON before writing headers to catch encoding errors early - Include DID in all repository error messages for debugging Logging improvements: - Replace log.Printf with slog.Error/Warn for structured logging - Consistent slog usage across handler, service, and repository layers Documentation improvements: - Update interface comments to list all 8 tables including votes - Fix CASCADE statement: "FK CASCADE deletes posts" (not votes) - Add authorization context to DeleteAccount service documentation Test additions: - DID with leading/trailing whitespace handling - Concurrent delete requests (race condition scenario) - Integration test verifying all 8 tables are cleaned up Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>