fix: correct and enrich 229 ISIN codes in equities database (#126)

Cross-referenced ISIN codes from multiple authoritative sources (ESMA regulatory data,
Borsa Italiana listings, top-rated Kaggle double-checked datasets) to fix incorrect and fill missing ISINs.

Changes:
- 82 ISIN corrections (replaced incorrect/obsolete codes)
- 147 ISIN additions (filled previously empty fields)
- Focused on MIL (Milan) exchange equities verified against Borsa Italiana official data
- Fixed 6 suspicious ISINs with country-code mismatches (e.g. non-IT ISINs for Italian companies)

PS
The situation with equities DB is rather complex. I have reason to suspect that many ISINs
are incorrect because they were extracted using overly simplistic or imprecise regex patterns
and/or because they were not properly joined/parsed using the "Symbol".

However, this is everything I was able to do using authoritative sources related to Borsa Italiana.

I believe the entire equities.csv database needs to be reviewed and thoroughly restructured.

Co-authored-by: carlo <carlo.ruggiero@pm.me>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 3 people