feat(notifications): document SMTP env and translate channel strings

Add the webhook + email Settings copy and the admin notification-health
panel labels across all six locales. Add the SMTP_* operator vars to the
env manifest (all-or-none: HOST + PORT + FROM enable the channel,
USER/PASS optional), to .env.production.example, and to the docker-compose
environment whitelist so an operator's .env values actually reach the
container.

.env.production.example

@@ -84,6 +84,24 @@ API_TOKEN_HMAC_KEY="<openssl rand -hex 32>"
 # APNS_KEY_FILE=""
 
 
+# -----------------------------------------------------------------------------
+# SMTP -- email notification channel (optional, all-or-none)
+# -----------------------------------------------------------------------------
+# Operator SMTP transport for the email channel. SMTP_HOST + SMTP_PORT +
+# SMTP_FROM together enable it; SMTP_USER/SMTP_PASS are optional (omit for an
+# unauthenticated relay). SMTP_SECURE=true uses implicit TLS (port 465);
+# unset/false uses STARTTLS (port 587). Per-user recipient address is set in
+# Settings -> Notifications, not here. Missing the core trio disables the
+# channel silently. Listed in docker-compose.yml's environment whitelist so
+# these reach the container.
+# SMTP_HOST=""
+# SMTP_PORT="587"
+# SMTP_FROM="HealthLog <noreply@example.com>"
+# SMTP_USER=""
+# SMTP_PASS=""
+# SMTP_SECURE="false"
+
+
 # -----------------------------------------------------------------------------
 # Deploy webhook -- Coolify -> HealthLog auto-deploy feedback
 # -----------------------------------------------------------------------------