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.

core/tei_client/__init__.py

@@ -0,0 +1,13 @@
+"""TEI Client — Athena API wrapper for Palladium."""
+
+from core.tei_client.client import AthenaAPIError, TEIClient
+from core.tei_client.models import TEIField, TEIReport, TEISummary, TEIValue
+
+__all__ = [
+    "AthenaAPIError",
+    "TEIClient",
+    "TEIField",
+    "TEIReport",
+    "TEISummary",
+    "TEIValue",
+]