docs: switch email provider recommendation from Mailgun to SendGrid

SendGrid handles both transactional emails and marketing campaigns
under one account. Updated alertmanager SMTP placeholders, hetzner
setup guide (Step 19.5), and environment reference to recommend
SendGrid as the primary email provider.

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

monitoring/alertmanager/alertmanager.yml

@@ -4,12 +4,13 @@
 global:
   resolve_timeout: 5m
 
-  # ─── SMTP Configuration ──────────────────────────────────────────────
-  # Fill in your SMTP credentials below
-  smtp_smarthost: 'smtp.example.com:587'       # TODO: Replace with your SMTP server
-  smtp_from: 'alerts@wizard.lu'                 # TODO: Replace with your sender address
-  smtp_auth_username: ''                        # TODO: Fill in SMTP username
-  smtp_auth_password: ''                        # TODO: Fill in SMTP password
+  # ─── SMTP Configuration (SendGrid) ──────────────────────────────────
+  # Sign up at sendgrid.com, create an API key, authenticate wizard.lu domain
+  # Username is literally the string "apikey", password is your SG.xxx API key
+  smtp_smarthost: 'smtp.sendgrid.net:587'       # SendGrid SMTP relay
+  smtp_from: 'alerts@wizard.lu'                 # Must match authenticated domain
+  smtp_auth_username: 'apikey'                  # Always "apikey" for SendGrid
+  smtp_auth_password: ''                        # TODO: Paste your SG.xxx API key here
   smtp_require_tls: true
 
 route: