[Data Source] Build St. Louis Board of Aldermen current-session scraper

[ThatXliner]

Goal

Add a current-session St. Louis Board of Aldermen scraper using official city pages, normalized into the shared local-government contract from #181.

Official sources

• Full-board agendas: https://www.stlouis-mo.gov/government/departments/aldermen/aldermanic-legislative-session.cfm
• Calendar: https://www.stlouis-mo.gov/government/departments/aldermen/events/
• Legislative information: https://www.stlouis-mo.gov/government/departments/aldermen/legislative-info.cfm

Scope

• Discover only the active aldermanic session (currently 2026–2027); do not crawl archives.
• Ingest full-board/committee meetings, agenda/minutes/packet URLs, Board Bills, resolutions, sponsors, titles, actions/status, and videos.
• Preserve stable agendaViewID/session/bill identifiers and source citations.
• Normalize into shared meeting/document/agenda-item/vote tables; serve cached data only.
• Do not use AI/OCR when structured HTML is available.

Acceptance criteria

• Fixtures/tests cover active-session discovery, agenda-to-bill linking, revised documents, sponsors, and session boundaries.
• Retry/backoff, low concurrency, hashes/versioning, idempotent upserts, env contract, and docs.
• Scraper/API/DB checks pass. No credentials required.