feat(data): Phase 2b step 1 — expand stg_regional_income + Census income extractor

- stg_regional_income.sql: accept NUTS-1 (3-char) + NUTS-2 (4-char) codes;
  rename nuts1_code → nuts_code; add nuts_level column; NUTS-2 rows were
  already in the landing zone but discarded by LENGTH(geo_code) = 3
- scripts/download_gisco_nuts.py: one-time download of GISCO NUTS-2 boundary
  GeoJSON (NUTS_RG_20M_2021_4326_LEVL_2.geojson, ~5MB) to landing zone;
  uncompressed because ST_Read cannot read .gz files
- census_usa_income.py: new extractor for ACS B19013_001E state-level median
  household income; follows census_usa.py pattern; 51 states + DC
- all.py + pyproject.toml: register census_usa_income extractor

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/download_gisco_nuts.py

@@ -0,0 +1,81 @@
+"""Download NUTS-2 boundary GeoJSON from Eurostat GISCO.
+
+One-time (or on NUTS revision) download of NUTS-2 boundary polygons used for
+spatial income resolution in dim_locations. Stored uncompressed because DuckDB's
+ST_Read function cannot read gzipped files.
+
+NUTS classification changes approximately every 7 years. Current revision: 2021.
+
+Output: {LANDING_DIR}/gisco/2024/01/nuts2_boundaries.geojson  (~5MB, uncompressed)
+
+Usage:
+    uv run python scripts/download_gisco_nuts.py [--landing-dir data/landing]
+
+Idempotent: skips download if the file already exists.
+"""
+
+import argparse
+import sys
+from pathlib import Path
+
+import niquests
+
+# NUTS 2021 revision, 20M scale (1:20,000,000), WGS84 (EPSG:4326), LEVL_2 only.
+# 20M resolution gives simplified polygons that are fast for point-in-polygon
+# matching without sacrificing accuracy at the NUTS-2 boundary level.
+GISCO_URL = (
+    "https://gisco-services.ec.europa.eu/distribution/v2/nuts/geojson/"
+    "NUTS_RG_20M_2021_4326_LEVL_2.geojson"
+)
+
+# Fixed partition: NUTS boundaries are a static reference file, not time-series data.
+# Use the NUTS revision year as the partition to make the source version explicit.
+DEST_REL_PATH = "gisco/2024/01/nuts2_boundaries.geojson"
+
+HTTP_TIMEOUT_SECONDS = 120
+
+
+def download_nuts_boundaries(landing_dir: Path) -> None:
+    dest = landing_dir / DEST_REL_PATH
+    if dest.exists():
+        print(f"Already exists (skipping): {dest}")
+        return
+
+    dest.parent.mkdir(parents=True, exist_ok=True)
+    print(f"Downloading NUTS-2 boundaries from GISCO...")
+    print(f"  URL: {GISCO_URL}")
+
+    with niquests.Session() as session:
+        resp = session.get(GISCO_URL, timeout=HTTP_TIMEOUT_SECONDS)
+        resp.raise_for_status()
+
+    content = resp.content
+    assert len(content) > 100_000, (
+        f"GeoJSON too small ({len(content)} bytes) — download may have failed"
+    )
+    assert b'"FeatureCollection"' in content, "Response does not look like GeoJSON"
+
+    # Write uncompressed — ST_Read requires a plain file
+    tmp = dest.with_suffix(".geojson.tmp")
+    tmp.write_bytes(content)
+    tmp.rename(dest)
+
+    size_mb = len(content) / 1_000_000
+    print(f"  Written: {dest} ({size_mb:.1f} MB)")
+    print("Done. Run SQLMesh plan to rebuild stg_nuts2_boundaries.")
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--landing-dir", default="data/landing", type=Path)
+    args = parser.parse_args()
+
+    if not args.landing_dir.is_dir():
+        print(f"Error: landing dir does not exist: {args.landing_dir}", file=sys.stderr)
+        sys.exit(1)
+
+    download_nuts_boundaries(args.landing_dir)
+
+
+if __name__ == "__main__":
+    main()