fix: RateLimit permission denied — code + deploy scripts

RateLimit.php:
- Silence mkdir() with @ operator
- Guard file_put_contents with is_writable() check (graceful degrade)

scripts/deploy-server.sh + setup-server.sh:
- mkdir -p storage/cache/rate_limit on every deploy
- chown www-data:posterg + chmod 2775 on storage/cache/
  so php-fpm can always write rate limit files

scripts/deploy-server.sh

@@ -25,7 +25,7 @@ printf "==================================\n\n"
 
 # ── Step 1: Permissions ───────────────────────────────────────────────────────
 printf "📋 Step 1: Fixing file permissions...\n"
-printf "--------------------------------------\n"
+echo "--------------------------------------"
 
 chown -R www-data:posterg /var/www/posterg/
 ok "Ownership: www-data:posterg"
@@ -50,7 +50,7 @@ ok "Cache dirs: created and owned by www-data:posterg"
 
 # ── Step 2: Nginx config ──────────────────────────────────────────────────────
 printf "\n📋 Step 2: Deploying nginx configuration...\n"
-printf "--------------------------------------------\n"
+echo "--------------------------------------------"
 
 if [ ! -f "/tmp/posterg.conf" ]; then
     err "/tmp/posterg.conf not found — run: just deploy-nginx"
@@ -73,7 +73,7 @@ fi
 
 # ── Step 3: Validate ──────────────────────────────────────────────────────────
 printf "\n📋 Step 3: Testing nginx configuration...\n"
-printf "------------------------------------------\n"
+echo "------------------------------------------"
 
 if nginx -t 2>&1; then
     ok "Nginx configuration is valid"