Make upload limit and Caddy settings configurable via env vars

- API: read the upload size limit from MAX_UPLOAD_MEGABYTES (default 15)
  and keep MAX_UPLOAD_BYTES as the byte value used by /predict
- Caddy: read the ACME email and request body limit from CADDY_EMAIL and
  CADDY_MAX_BODY (default 16MB, ~1 MB above the API limit)
- compose: pass the new variables to caddy_proxy and cnn_api

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

caddy/Caddyfile

@@ -1,5 +1,5 @@
 {
-	email marvin.krausser277@gmail.com
+	email {$CADDY_EMAIL:marvin.krausser277@gmail.com}
 }
 
 (common) {
@@ -25,7 +25,7 @@ marvinkrausser.com {
 api.marvinkrausser.com {
 	import common
 	request_body {
-		max_size 10MB
+		max_size {$CADDY_MAX_BODY:16MB}
 	}
 	reverse_proxy cnn_api:8000
 }