refactor: move credentials to .env and add python-dotenv support

Replace hardcoded database credentials, ntfy tokens, and Uptime Kuma
push URLs with environment variable lookups via python-dotenv.
Add .env.example template and update .gitignore to exclude .env.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,15 @@
+# SQL Server
+DB_DRIVER=ODBC Driver 18 for SQL Server
+DB_SERVER=
+DB_DATABASE=
+DB_USERNAME=
+DB_PASSWORD=
+
+# ntfy
+NTFY_SERVER_URL=
+NTFY_TOPIC=
+NTFY_TOKEN=
+
+# Uptime Kuma
+UPTIME_KUMA_PUSH_URL=
+EXCEL_SYNC_UPTIME_KUMA_PUSH_URL=