Fix: oldest report rows dropped on every weekly Action run

[Copilot]

The GitHub traffic API returns a rolling 14-day window. Each weekly run regenerated all_periods solely from the fresh API response, so the oldest week — now outside the window — was permanently deleted from the TSV on every run.

Changes

• src/reports/base.py — union existing_data.keys() into all_periods so recorded periods are never silently dropped when they age out of the API window:

# Before
all_periods = {p for data in entity_data.values() for p in data.keys()}

# After
all_periods = {p for data in entity_data.values() for p in data.keys()} | set(existing_data.keys())

• tests/test_report.py — two regression tests (TestReportPreservesExistingPeriods) that reproduce the weekly-run scenario: one verifying the aged-out week remains in the report, one verifying its stored value is unchanged.

Original prompt

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This section details on the original issue you should resolve

<issue_title>GitHub Action workflows seem to always remove the first line of the report tables</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes GitHub Action workflows seem to always remove the first line of the report tables #12

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