fix: unify OpenDART env var name with Gitea Secrets; add missed migration header notes

tools/ingest_fundamental_raw.py read DART_API_KEY, but the Gitea Secret
is registered as OPENDART_OPENAPI_KEY, and no workflow bridges the two
(none currently invoke this script). Renamed the code side to match
the secret name directly rather than adding a mapping layer, so
whenever this gets wired into a workflow it just works. Updated the
matching README setup instructions.

Also includes the V9/V10 migration header explanations (why they were
renamed from V003/V004) that were written earlier but missed from the
previous commit's file list.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/dotnet/QuantEngine.Infrastructure/Migrations/V9__Add_Audit_Trail_Tables.sql

@@ -1,7 +1,18 @@
--- Migration: V003_add_audit_trail_tables.sql
+-- Migration: V9__Add_Audit_Trail_Tables.sql (renamed from V003_add_audit_trail_tables.sql
+-- on 2026-07-30 — see header note below)
 -- Purpose: Add audit trail tables for tracking all data changes
 -- Date: 2026-07-24
 -- Status: APPROVED for Phase 0 implementation
+--
+-- 2026-07-30: Originally named V003_add_audit_trail_tables.sql, which alphabetically sorted
+-- BEFORE V1__Initial_Schema.sql. Its trigger-creation guards (IF EXISTS checks on
+-- kis_collection_runs / kis_collection_snapshots) would have silently no-op'd forever on a
+-- fresh database, since those tables (created by V2) wouldn't exist yet when V003 ran first.
+-- Renamed to V9 (after DbMigrator.cs was given a numeric-aware script comparer,
+-- MigrationScriptNameComparer, so "V9"/"V10" sort correctly relative to "V1".."V8" regardless
+-- of digit count) so it now runs after its dependencies exist. Also fixed a MySQL-only inline
+-- INDEX syntax that made this script fail outright on PostgreSQL (separate fix, same day).
+-- Confirmed via production query that this migration had never actually applied before either fix.
 
 -- ============================================================================
 -- kis_collection_runs_audit: Audit trail for collection runs