Use unique temp PNG paths and fix crossfade padding handling

[AJaySi]

Motivation

• Prevent collisions when multiple processes render temporary chart/card/bullet PNGs concurrently by generating unique temp filenames.
• Preserve fractional fade durations in crossfade_concat by avoiding an integer cast that could truncate fade_dur.

Description

• Added imports for tempfile and uuid and introduced helper _temp_png(prefix) to return a unique temp PNG path.
• Replaced hard-coded /tmp/*.png intermediate paths with calls to _temp_png for insight_card, bullets, and chart.
• Updated crossfade_concat to use padding=-fade_dur instead of -int(fade_dur) so fade durations remain precise.
• Minor newline/formatting cleanup at end of file.

Testing

• Ran the module demo via python backend/services/podcast/broll_composer.py to generate example bullet and insight card assets and observed successful output.

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Codex Task