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>

.env.example

@@ -0,0 +1,18 @@
+# Copy this file to .env and fill in your real values.
+# .env is gitignored and is the only place secrets should live.
+
+SMTP_SERVER=smtp.example.com
+SMTP_PORT=25
+SENDER_EMAIL=santa@example.com
+CSV_PATH=path/to/your/csv/files
+CSV_PREFIX=secret_santa_DB
+HISTORY_YEARS=2
+DRAW_PER_PERSON=2
+
+# Optional overrides — {name}/{draws}/{year} placeholders are filled in at send time.
+# EMAIL_SUBJECT=Secret Santa {year} Draw
+# EMAIL_BODY="Hello {name},\n\nYou have been chosen to give gifts to: {draws}.\n"
+
+# Web GUI basic-auth (leave unset to disable auth — not recommended outside localhost)
+WEBAPP_USERNAME=admin
+WEBAPP_PASSWORD=change-me