feat: add zip/postal code support

[plc]

Summary

I live in a small town that has the name of a big city so the city/country inputs kept getting the wrong place. I asked Claude to add zip code support and hopefully it did it responsibly.

• Adds --zipcode / -z CLI argument to generate posters from a zip or postal code instead of requiring --city and --country
• Geocodes via Nominatim with country_codes parameter to ensure accurate results (defaults to US)
• Auto-resolves city and country names from the postal code for poster labels and filenames
• Users can still override with --city or --country if the auto-resolved names aren't ideal

Usage

# US zip code (defaults to US)
python create_map_poster.py -z 10001 -t noir -d 12000

# Non-US postal code (specify country)
python create_map_poster.py -z "SW1A 1AA" -C "UK" -t noir -d 15000

# Override the auto-resolved city name
python create_map_poster.py -z 80027 -c "Louisville" -t terracotta

Test plan

• --help shows new --zipcode argument
• US zip code resolves correctly (tested 80027 → Louisville, CO)
• Flake8 passes (strict and full, max-line-length 160)
• mypy passes (no new errors)
• Syntax validation passes
• Non-US postal code with -C resolves correctly (SW1A 1AA → City of Westminster, UK)
• Invalid zip code produces clear error message (tested "ZZZZZ")
• Explicit --latitude/--longitude still takes priority over zip coords

🤖 Generated with Claude Code

[PasikVN]

Real question: Is it not enough to use zip code as city name + country name?

python create_map_poster.py -c "80027" -C "USA" -t terracotta