Extract shared SECHttpClient from ETF fetcher (retry, backoff, cache, throttle)
and apply it to both ETF and core SEC services, fixing missing rate limiting.
Add SECFiling DB model, Pydantic schemas, SECFilingsService with auto-indexing,
and REST endpoints at /filings/search, /filings/documents, /filings/exhibit.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>