Pull through additional OSM and Overture metadata fields.

[njhenry]

Pulls through richer per-POI metadata from both OSM and Overture into the conflated snapshot, plus one minor bug fix related to brand in the conflated dataset.

File changes

Overture extraction (src/openpois/io/overture.py)

• DuckDB SELECT now grabs addresses[1].{freeform, locality, region, postcode, country}, websites, phones, socials, and categories.alternate, with the same column names propagated through _finalize_snapshot_in_duckdb.

OSM extraction (config.yaml)

• Adds access to download.osm.extract_keys so it survives into the snapshot.

Merge (src/openpois/conflation/merge.py)

• New helpers _present, _blend_with_backfill, _col_or_null, _unwrap_first_element consolidate the column-guard / blend / list-unwrap logic that was previously inline.
• Blend semantics are now bidirectional: the higher-confidence source still wins, but if its value is null/empty the merge falls back to the other side instead of leaving the cell null. Empty strings ("") are treated as missing — Overture sometimes emits them.
• Matched rows now carry:
  • Blended primaries: addr_street, addr_city, addr_state, addr_postcode, addr_country, phone, website.
  • OSM-only: addr_housenumber, addr_unit, opening_hours, access.
  • Overture-only: overture_socials, overture_categories_alternate.
  • Per-source traceability: osm_addr_*, overture_addr_*, osm_phone, overture_phones, osm_website, overture_websites.
• _build_unmatched_osm_gdf and _build_unmatched_overture_gdf populate the same schema with appropriate nulls so the three sub-frames concat cleanly.

Conflation driver (scripts/conflation/conflate.py)

• Adds the new metadata columns to OSM_MERGE_COLS / OVERTURE_MERGE_COLS so they survive the per-shared-label row slice.

Tests (tests/test_merge.py)

• New TestMetadataPullThrough class (9 tests) covers: matched-row blending, OSM-only fields, unmatched-side traceability, LIST<STRING> unwrap (including empty list → None), access flow, and two regressions — brand backfill when OSM is higher-confidence but null, and empty-string-as-missing.
• Existing matched-schema assertion extended to include every new column.

Bug fix:

Before this PR, when OSM had higher confidence than Overture but a null brand, the matched row's brand was dropped instead of falling back to Overture. _blend_with_backfill makes that backfill symmetric and is now used for name, brand, every addr_*, phone, and website.