feat(sql): add the postcode as a PostgreSQL domain and DuckDB macros

[Adeniyikayodee]

The postcode as a column type, in plain SQL. There is no extension to build, so both files load on managed services that accept none.

• PostgreSQL (sql/postgres.sql): an ng_postcode domain that refuses anything but a compact code, an ng_postcode_prefix domain, and six immutable functions: parse a code or a prefix, format, truncate to a segment, parent, and within.
• DuckDB (sql/duckdb.sql): macros with the same names and answers. DuckDB has no domains, so the README gives the constraint to use.
• Same answers as the libraries: scripts/sql_conformance.py turns the cases in spec/vectors.json into SQL and runs them, 101 on each engine. Two steps in the python job run it, so the ruleset is unchanged.
• Locale: the SQL uses translate() because upper() follows the database locale. I ran PostgreSQL 17 under a Turkish locale, where a naive version would mishandle i, along with PostgreSQL 14 and DuckDB 1.5.
• Index use: selecting a district as a range on the column uses its index; I confirmed that with EXPLAIN and the README shows the form.

One thing the README warns about because I got it wrong first: a DuckDB constraint written with = lets a malformed code through, since comparing with NULL is not false. It needs IS NOT DISTINCT FROM.

The CI steps use the runner's preinstalled PostgreSQL, which I could not exercise locally. If they fail here, I will fix them on this branch.