Add authenticated history viewer

.env.example

@@ -1,6 +1,9 @@
 # Create with: cp .env.example .env ; then replace the placeholder.
 # Generate: python3 -c "import secrets; print(secrets.token_urlsafe(32))"
 API_TOKEN=REPLACE_WITH_A_RANDOM_32+_CHARACTER_TOKEN
+# Login for the browser viewer at /; replace both values with long random secrets.
+VIEWER_USERNAME=admin
+VIEWER_PASSWORD=REPLACE_WITH_ANOTHER_RANDOM_32+_CHARACTER_SECRET
 # Optional. Leave empty to keep all data only in SQLite.
 DISCORD_WEBHOOK_URL=
 DATA_DIR=/data