feat(alerts): Benachrichtigungen per E-Mail und Webhook (#012)

SMTP mit STARTTLS fuer Microsoft 365, Webhook mit HMAC-Signatur, taegliche oder
woechentliche Zusammenfassung, Versandprotokoll und Testversand. Keine
Doppelmeldungen dank Pruefung gegen das Protokoll. 134 Tests.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "proxledger",
-  "version": "0.7.0",
+  "version": "0.8.0",
   "private": true,
   "scripts": {
     "dev": "next dev",
@@ -32,12 +32,14 @@
     "bootstrap-icons": "^1.13.1",
     "croner": "^10.0.1",
     "next": "16.3.5",
+    "nodemailer": "^10.0.10",
     "react": "19.2.8",
     "react-dom": "19.2.8",
     "zod": "^4.6.5"
   },
   "devDependencies": {
     "@types/node": "^24.13.5",
+    "@types/nodemailer": "^8.0.2",
     "@types/react": "^19",
     "@types/react-dom": "^19",
     "dotenv": "^17.4.2",