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>

tools/ingest_fundamental_raw.py

@@ -52,7 +52,8 @@ DEFAULT_JSON = ROOT / "GatherTradingData.json"
 DEFAULT_OUT = TEMP = ROOT / "Temp" / "fundamental_raw_v1.json"
 
 # API Keys
-DART_API_KEY = os.environ.get("DART_API_KEY")
+# 환경변수명은 Gitea Secrets에 이미 등록된 OPENDART_OPENAPI_KEY와 통일한다 (2026-07-30).
+DART_API_KEY = os.environ.get("OPENDART_OPENAPI_KEY")
 DART_CORP_MAP_CACHE = TEMP / "dart_corp_map.json"
 
 # ETF 식별자 패턴