Add exclusion support, multi-year history helper, and remove secrets from env.py

- draw.py/file_io.py/main.py: wire in exclusion pairs so the draw can avoid
  configured giver/receiver combinations.
- utils.py: add get_last_n_years helper.
- env.py: load configuration from environment variables (via .env) instead of
  hardcoding real SMTP/CSV secrets in a tracked file; add .env.example as the
  template and gitignore .env.
- test_draws.py: drop hardcoded personal path, use env.py config instead.
- add requirements.txt for the upcoming web GUI/container work.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitattributes

@@ -0,0 +1,2 @@
+# Specify filepatterns you want to assign special attributes.
+