Preserve crossfade precision, use per-scene temp assets, and dedupe line-trend chart

[AJaySi]

Motivation

• Ensure crossfade overlap uses the requested fractional duration instead of truncating to an integer so transitions match the configured fade_dur precisely.
• Avoid collisions and accidental asset reuse by replacing fixed /tmp/*.png intermediate filenames with unique, per-scene asset paths.
• Remove a duplicate make_line_trend implementation and keep a single canonical, configurable version to avoid divergence and enable marker/area options.

Description

• Updated crossfade_concat to call concatenate_videoclips(..., padding=-fade_dur) to preserve float precision for crossfade padding.
• Introduced scene-scoped temporary directories and UUID-named asset files for intermediate PNGs; threaded a temp_dir through dispatch_scene, build_data_scene, and build_bullet_scene so chart/overlay assets are written into a job-specific directory.
• Removed the duplicate make_line_trend and kept the canonical version which accepts show_area and show_markers options.
• Updated pipeline_from_json to create a per-scene temp directory and pass it into dispatch_scene.
• Updated BrollService.generate_scene_broll to create a unique scene_assets_<id>_<rand> directory and pass it as temp_dir when invoking the composer, and imported Insight, SceneAssets, dispatch_scene, and compose_video from the composer module.

Testing

• Compiled both modified modules with python -m compileall backend/services/podcast/broll_composer.py backend/services/podcast/broll_service.py, which succeeded.

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Codex Task