Add stereo and bass enhance APIs via PEQ overlay.

Expose POST /api/audio/stereo-enhance and bass-enhance with 0–100 levels,
persist enhancements in AudioProfile, and document the virtual EQ mapping in
the manual and SigmaStudio chapter.

Co-authored-by: Cursor <cursoragent@cursor.com>

Software/docs/manual/ch-sigmastudio.tex

@@ -213,6 +213,26 @@ SigmaStudio export (Section~\ref{sec:ss-export}).
   \label{tab:ss-runtime}
 \end{table}
 
+\section{Virtual enhancements (stereo depth and bass boost)}
+\label{sec:ss-enhancements}
+
+The SigmaStudio export does not include dedicated stereo widener or bass
+boost blocks. Firmware~0.5.0 maps enhancement levels (0--100) onto the
+existing Param EQ1 bands at runtime:
+
+\begin{itemize}
+  \item \textbf{Bass enhance} --- peaking boost at 100\,Hz (+9\,dB max)
+        and 400\,Hz (+3\,dB max).
+  \item \textbf{Stereo enhance} --- slight 1\,kHz cut, plus 3\,kHz and
+        8\,kHz lift for a wider, more present image. This is a
+        psychoacoustic curve, not mid/side processing.
+\end{itemize}
+
+Enhancement levels are stored in \texttt{AudioProfile::enhancements} and
+applied by \texttt{core::applyEnhancementsToEq()} before safeload. Base EQ
+band settings in NVS are preserved; overlays replace affected bands only
+while the corresponding level is greater than zero.
+
 \begin{drcaution}[Use safeload for live changes]
 All runtime updates to the volume, source levels, and EQ bands must go
 through the ADAU1701 safeload mechanism. Writing parameter cells directly