Redirect emails to dev address in non-production; fix PAID stamp color

- EmailService: add RedirectIfNonProd() mirroring SmsService pattern;
  reads SendGrid:DevRedirectEmail and redirects all outbound email in
  non-production so real customers are never contacted on local/dev
- appsettings.json: set DevRedirectEmail to spouliot@scppowdercoating.com
- PdfService: revert Opacity() (not in QuestPDF 2024.12.3); use
  Colors.Green.Lighten2 for stamp + border to achieve lighter look

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/appsettings.json

@@ -45,7 +45,8 @@
   "SendGrid": {
     "ApiKey": "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",
     "FromEmail": "spouliot@scppowdercoating.com",
-    "FromName": "Powder Coating App Staff"
+    "FromName": "Powder Coating App Staff",
+    "DevRedirectEmail": "spouliot@scppowdercoating.com"
   },
   "Twilio": {
     "AccountSid": "your-twilio-account-sid",