docs: correct historical coverage to the canonical 2017-01-03 / 80B figures

Every public surface advertised a dataset start of 2018-04-16 and 6.7B option
rows. Both are wrong and both understate the product. Verified against the live
coverage endpoint: SPY runs from 2017-01-03, and the archive holds 80B+ option
rows across 26 fully-ramped symbols. Per-symbol first dates still vary, so the
wording says SPY rather than implying a universal start.

That is 15 extra months of history and more than 10x the row count that the
published SDKs were claiming.

Test fixtures that pass 2018-04-16 to a mocked 404 are deliberately untouched:
the date there is arbitrary input to an error-mapping assertion, not a coverage
claim, so rewriting it would be churn with no meaning.

Author: tdobrowolski1

README.md

@@ -283,7 +283,7 @@ calculator and stock quotes. Paid tiers add:
 - **DEX, VEX (vanna) and CHEX (charm) exposure, plus max pain** — from the **Basic tier**
   ($79/mo), with ETF and index symbols.
 - **Full-chain GEX, 0DTE and flow analytics** — from the **Growth tier** ($299/mo).
-- **Point-in-time replay since 2018, SVI vol surfaces, VRP analytics, higher-order Greeks**,
+- **Point-in-time replay since 2017, SVI vol surfaces, VRP analytics, higher-order Greeks**,
   uncached and unlimited — the **Alpha tier** ($1,499/mo). FlashAlpha is one of the only
   public APIs publishing aggregate vanna and charm exposure across the full universe, with
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