feat: Add configurable proxy for outgoing traffic to prevent SSRF

docker-compose-dev.yml

@@ -50,6 +50,10 @@ services:
       # It should not be enabled without a rate limiter for security reasons.
       # - SHOW_DETAILED_AUTH_ERRORS=false
 
+      # All outgoing HTTP requests (SMTP, webhooks, Apprise notifications, favicon fetching, etc.)
+      # will be sent through this proxy if set.
+      # - OUTGOING_PROXY=http://proxy:3128
+
       # - S3_ENDPOINT=
       # - S3_REGION=
       # - S3_ACCESS_KEY_ID=