[Data Source] Ingest Missouri SOS current-cycle candidates, ballot measures, and results

[ThatXliner]

Goal

Add a current-cycle-only Missouri election scraper and API reader using official Missouri Secretary of State sources.

Official sources

• 2026 certified candidate filing: https://s1.sos.mo.gov/candidatesonweb/
• 2026 certified ballot measures: https://www.sos.mo.gov/petitions/2026BallotMeasures
• ShowMO Votes/results entry point: https://www.sos.mo.gov/elections/showmovotes
• 2026 calendar: https://www.sos.mo.gov/elections/calendar/2026cal

Scope

• Discover the active primary/general election rather than hard-coding archival URLs.
• Parse certified candidates in ballot order: office, district, party, name, and withdrawal status.
• Parse only certified 2026 measures: official title, fair ballot language, fiscal statement, election date, full-text/certificate URLs, and provenance.
• Ingest official results when published; before election night, fail soft with a clear unavailable diagnostic.
• Persist through the provider-neutral election model from Add North Carolina and Texas civic data scrapers #181 with checksums, source versions, certification status, and citations.
• Exclude candidate addresses/contact information and all voter-level data. Do not expose historical-cycle browsing.

Acceptance criteria

• Deterministic fixtures/tests cover ballot order, withdrawals, parties/districts, certified measures, and unavailable/updated results.
• Shared retry/backoff, respectful concurrency, idempotent upserts, max-items env contract, API reader, and documentation.
• Scraper/API/DB tests and typechecks pass.

No API key is required.