Refactor chart preview flow to reuse a single chart ID

[AJaySi]

Motivation

• Ensure the same chart_id is used end-to-end for chart preview generation, URL construction, and serving so preview lookup is deterministic and cannot fail due to mismatched IDs.
• Avoid generating different internal IDs in the service and the API handler which caused inconsistent preview URLs and retrieval errors.

Description

• Generate the chart_id once in the preview handler and pass it into BrollService.generate_chart_preview(...) so the service writes the file with the same ID. (backend/api/podcast/handlers/broll.py).
• Add deterministic helpers get_chart_preview_filename() and get_chart_preview_path() to BrollService, and make generate_chart_preview(...) accept an optional chart_id to allow controlled filenames. (backend/services/podcast/broll_service.py).
• Construct the preview URL using the returned file basename while retaining the shared chart_id, and validate the incoming filename in serve_chart_preview() by comparing it to get_chart_preview_filename(chart_id) before serving. (backend/api/podcast/handlers/broll.py).

Testing

• Ran python -m py_compile backend/api/podcast/handlers/broll.py backend/services/podcast/broll_service.py which completed successfully.

---

Codex Task