Expose SEC ACCEPTANCE-DATETIME so downstream consumers (filing_poller)
can populate Document.accepted_at_utc and Event.filed_at_utc.
- SECFiling model: add nullable accepted_at TIMESTAMP column
- main.py startup: ALTER TABLE migration for existing SQLite DBs
- sec_filings_service: extract acceptanceDateTime from SEC JSON, store on
INSERT and force_refresh UPDATE
- FilingSummary schema: add Optional[str] accepted_at field
- filings endpoint: map accepted_at as ISO 8601 string in response
- CHANGELOG: v3.0.1 entry
Existing rows have accepted_at=NULL; backfill with force_refresh=true.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>