* fix: change postal_code column from integer to string
Allows non-numeric postal codes such as UK format (e.g. SW1A 2AA).
The integer column was silently dropping any alphanumeric input.
The migration is marked irreversible — once alphanumeric postal codes
exist, they cannot be safely cast back to integer.
* fix: update schema.rb, quote fixture postal_code, and add alphanumeric test
* fix: use conventional migration timestamp
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