Remove matplotlib display usage from plot_spectrogram

[Copilot]

plot_spectrogram() in spectrogram_utils.py was calling plt.show() to display plots interactively when no output path was provided. This breaks headless environments and violates the principle of generating images without display.

Changes

• Removed interactive display: Eliminated plt.show() call, always saves to file
• Force non-GUI backend: Added matplotlib.use('Agg') at module import
• Temporary file handling: When plotpath=None, creates temp file with NamedTemporaryFile(delete=False)
• API change: Return signature changed from ims to (ims, plotpath) tuple

Example

# Before - would show plot window if plotpath=None
ims = plot_spectrogram('audio.wav')  # plt.show() called

# After - always saves, returns path
ims, path = plot_spectrogram('audio.wav')  # temp file created
ims, path = plot_spectrogram('audio.wav', 'output.png')  # explicit path

Breaking change: Function signature modified. Verified unused in codebase.

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