Move the hard-coded ALIASES dict into resources/aliases.toml (loaded with
stdlib tomllib), expand it with agencies, international bodies, and country
names, and fix two correctness bugs in the naive replace():

- Whole-word matching so 'united states' no longer collapses inside
  'united statesman', and 'inflation rate' no longer mangles 'inflation rates'.
- Longest-phrase-first application so 'president of the united states'
  resolves to 'potus' before 'united states' fires.
- Correct a stopword collision: 'united nations' -> 'un' was silently dropped
  because 'un' is a stopword; it now maps to 'unitednations'.

Aliases are threaded through normalize_headline -> is_headline ->
_build_headlines -> prepare_headlines and loaded in main.py. Includes unit
tests and README coverage.