removed sensor fallback range

[haoruizhou]

• data-downloader/backend/services.py:71 now feeds the sensor collector with fallback start/end timestamps derived from the freshly scanned run windows, eliminating the static date dependency and ensuring both scanners stay in sync; the helper at data-downloader/backend/services.py:124 safely scans the windows, normalizes them to UTC, and only returns a range when it is valid so the SQL chunking logic in backend/ sql.py stays unchanged and memory-friendly.
• data-downloader/backend/config.py:31 drops the unused SENSOR_FALLBACK_* settings so the app no longer advertises manual fallback tuning knobs that would fight the new automatic behavior.
• data-downloader/README.md:35 explains the new automatic fallback derived from the date scanner, and data- downloader/.env.example:11 removes the obsolete environment variables from the sample configuration to keep deploys aligned with the new flow.

[Copilot]

Pull Request Overview

This PR replaces the static sensor fallback date configuration with an automatic fallback mechanism that derives start/end timestamps from the scanned run windows. This ensures the run scanner and sensor collector remain synchronized without manual date configuration.

• Implements _build_sensor_fallback_range helper to compute fallback dates from scanned runs
• Removes SENSOR_FALLBACK_START and SENSOR_FALLBACK_END configuration settings
• Updates documentation to reflect the new automatic behavior

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
installer/data-downloader/backend/services.py	Adds helper method to build sensor fallback range from scanned runs and integrates it into the scan workflow
installer/data-downloader/backend/config.py	Removes obsolete sensor fallback configuration fields
installer/data-downloader/README.md	Updates documentation to explain the new automatic fallback behavior
installer/data-downloader/.env.example	Removes deprecated environment variable examples

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