docs(inventory): document population pipeline implementation findings

- Add section 9 to data-sources-inventory.md covering live API quirks:
  Eurostat SDMX city labels response shape, ONS CSV download path (observations
  API 404s), US Census ACS place endpoint, GeoNames cities15000 bulk format
- Add population coverage summary table and DuckDB glob limitation note
- fix(extract): census_usa + geonames write empty placeholder when credentials
  absent so SQLMesh staging models don't fail with "no files found"

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

extract/padelnomics_extract/src/padelnomics_extract/census_usa.py

@@ -64,7 +64,12 @@ def extract(
     """Fetch ACS 5-year place population. Skips if already run this month."""
     api_key = os.environ.get("CENSUS_API_KEY", "").strip()
     if not api_key:
-        logger.warning("CENSUS_API_KEY not set — skipping US Census extract")
+        logger.warning("CENSUS_API_KEY not set — writing empty placeholder so SQLMesh models can run")
+        year, month = year_month.split("/")
+        dest_dir = landing_path(landing_dir, "census_usa", year, month)
+        dest = dest_dir / "acs5_places.json.gz"
+        if not dest.exists():
+            write_gzip_atomic(dest, b'{"rows": [], "count": 0}')
         return {"files_written": 0, "files_skipped": 1, "bytes_written": 0}
 
     # Skip if we already have data for this month (annual data, monthly cursor)

extract/padelnomics_extract/src/padelnomics_extract/geonames.py

@@ -105,7 +105,12 @@ def extract(
     """Download GeoNames cities15000.zip. Skips if already run this month."""
     username = os.environ.get("GEONAMES_USERNAME", "").strip()
     if not username:
-        logger.warning("GEONAMES_USERNAME not set — skipping GeoNames extract")
+        logger.warning("GEONAMES_USERNAME not set — writing empty placeholder so SQLMesh models can run")
+        year, month = year_month.split("/")
+        dest_dir = landing_path(landing_dir, "geonames", year, month)
+        dest = dest_dir / "cities_global.json.gz"
+        if not dest.exists():
+            write_gzip_atomic(dest, b'{"rows": [], "count": 0}')
         return {"files_written": 0, "files_skipped": 1, "bytes_written": 0}
 
     last_cursor = get_last_cursor(conn, EXTRACTOR_NAME)