refactor: restructure repo into core/app modules with per-study folders

Reorganize Palladium codebase into a modular architecture with `core/`
shared logic and `app/` Streamlit UI, separating per-study assets into
`studies/YYYYMM_<Vendor>/` folders containing notebooks, seed data, and
configuration. Update README to reflect new structure, add `.gitignore`
entries for `.env` and study exports, and refresh component documentation.

.env.example

@@ -0,0 +1,10 @@
+# Athena API
+ATHENA_BASE_URL=https://athena.nttdata.com
+ATHENA_API_KEY=your-api-key-here
+
+# Optional — pre-set the active study + tool so notebooks/CLI pick them up
+# without editing config.py.
+#
+# PALLADIUM_REPORT_PUBLIC_ID=
+# PALLADIUM_TOOL_PUBLIC_ID=
+# PALLADIUM_PROPOSAL_ID=