Add official US and European collection and calendar storage

Collect official RSS and Atom summaries, preserve historical captures, and poll macro releases with bounded cadence and Retry-After handling. Separate news collectors from the economic calendar worker.

Track statistical periods, publication stages, first observations, revisions, and current official series. Include explicit migration files and isolated archive, parsing, and import tests; no migration or import is executed by this commit.

Keep dependency declarations and lockfile together for PDF archives and the upcoming captcha integration. Verify the staged snapshot with 90 focused tests.

backend/tests/fixtures/europe/ons_schedule.html

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+<html><body><a href="/releases/greenhousegasemissionsukprovisionalestimates2025" class="ons-u-fs-m ons-u-td-no ons-u-d-b" data-gtm-release-title="Greenhouse gas emissions, UK: provisional estimates, 2025" data-gtm-release-url="/releases/greenhousegasemissionsukprovisionalestimates2025" data-gtm-release-date="20261028" data-gtm-release-time="09:30" data-gtm-release-calendar-page="1" data-gtm-release-calendar-position="1">Greenhouse gas emissions, UK: provisional estimates, 2025</a><a href="/releases/deathsregisteredweeklyinenglandandwalesprovisionalweekending16october2026" class="ons-u-fs-m ons-u-td-no ons-u-d-b" data-gtm-release-title="Deaths registered weekly in England and Wales, provisional: week ending 16 October 2026" data-gtm-release-url="/releases/deathsregisteredweeklyinenglandandwalesprovisionalweekending16october2026" data-gtm-release-date="20261028" data-gtm-release-time="09:30" data-gtm-release-calendar-page="1" data-gtm-release-calendar-position="2">Deaths registered weekly in England and Wales, provisional: week ending 16 October 2026</a><a href="/releases/economicactivityandsocialchangeintheukrealtimeindicators29october2026" class="ons-u-fs-m ons-u-td-no ons-u-d-b" data-gtm-release-title="Economic activity and social change in the UK, real-time indicators: 29 October 2026" data-gtm-release-url="/releases/economicactivityandsocialchangeintheukrealtimeindicators29october2026" data-gtm-release-date="20261029" data-gtm-release-time="09:30" data-gtm-release-calendar-page="1" data-gtm-release-calendar-position="3">Economic activity and social change in the UK, real-time indicators: 29 October 2026</a><a href="/releases/businessdemographyquarterlyukjulytoseptember2026" class="ons-u-fs-m ons-u-td-no ons-u-d-b" data-gtm-release-title="Business demography, quarterly, UK: July to September 2026" data-gtm-release-url="/releases/businessdemographyquarterlyukjulytoseptember2026" data-gtm-release-date="20261029" data-gtm-release-time="09:30" data-gtm-release-calendar-page="1" data-gtm-release-calendar-position="4">Business demography, quarterly, UK: July to September 2026</a><a href="/releases/uknationalaccountsthebluebook2026" class="ons-u-fs-m ons-u-td-no ons-u-d-b" data-gtm-release-title="UK National Accounts, The Blue Book: 2026" data-gtm-release-url="/releases/uknationalaccountsthebluebook2026" data-gtm-release-date="20261030" data-gtm-release-time="09:30" data-gtm-release-calendar-page="1" data-gtm-release-calendar-position="5">UK National Accounts, The Blue Book: 2026</a><a href="/releases/grossfixedcapitalformationdeflatorcatalogue" class="ons-u-fs-m ons-u-td-no ons-u-d-b" data-gtm-release-title="Gross Fixed Capital Formation Deflator Catalogue" data-gtm-release-url="/releases/grossfixedcapitalformationdeflatorcatalogue" data-gtm-release-date="20261030" data-gtm-release-time="09:30" data-gtm-release-calendar-page="1" data-gtm-release-calendar-position="6">Gross Fixed Capital Formation Deflator Catalogue</a><a href="/releases/cpatocoicopconverterforhouseholdfinalconsumptionexpenditure" class="ons-u-fs-m ons-u-td-no ons-u-d-b" data-gtm-release-title="CPA to COICOP converter for household final consumption expenditure" data-gtm-release-url="/releases/cpatocoicopconverterforhouseholdfinalconsumptionexpenditure" data-gtm-release-date="20261030" data-gtm-release-time="09:30" data-gtm-release-calendar-page="1" data-gtm-release-calendar-position="7">CPA to COICOP converter for household final consumption expenditure</a><a href="/releases/inputoutputsupplyandusetables2026edition" class="ons-u-fs-m ons-u-td-no ons-u-d-b" data-gtm-release-title="Input-output supply and use tables: 2026 edition" data-gtm-release-url="/releases/inputoutputsupplyandusetables2026edition" data-gtm-release-date="20261030" data-gtm-release-time="09:30" data-gtm-release-calendar-page="1" data-gtm-release-calendar-position="8">Input-output supply and use tables: 2026 edition</a><a href="/releases/gdprevisionsinbluebook2026" class="ons-u-fs-m ons-u-td-no ons-u-d-b" data-gtm-release-title="GDP revisions in Blue Book: 2026" data-gtm-release-url="/releases/gdprevisionsinbluebook2026" data-gtm-release-date="20261030" data-gtm-release-time="09:30" data-gtm-release-calendar-page="1" data-gtm-release-calendar-position="9">GDP revisions in Blue Book: 2026</a><a href="/releases/annualgrossfixedcapitalformationbyindustryandassetoctober2026" class="ons-u-fs-m ons-u-td-no ons-u-d-b" data-gtm-release-title="Annual gross fixed capital formation by industry and asset: October 2026" data-gtm-release-url="/releases/annualgrossfixedcapitalformationbyindustryandassetoctober2026" data-gtm-release-date="20261030" data-gtm-release-time="09:30" data-gtm-release-calendar-page="1" data-gtm-release-calendar-position="10">Annual gross fixed capital formation by industry and asset: October 2026</a><a href="/releases/supplyandusetablesdatasourcescatalogue2026" class="ons-u-fs-m ons-u-td-no ons-u-d-b" data-gtm-release-title=" Supply and use tables data sources cataLine truncated