feat(PROJ-27): Docker-Support + install.sh v2.0 (native + Docker-Modus)

Adds multi-stage Dockerfiles (Go+Xapian CGO, Next.js standalone),
docker-compose.yml, webhook-basierter Deploy-Flow und erweitertes
install.sh mit interaktiver Modus-Auswahl.

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

scripts/webhook-hooks.json

@@ -0,0 +1,32 @@
+[
+  {
+    "id": "deploy",
+    "execute-command": "/opt/archivmail/scripts/deploy.sh",
+    "command-working-directory": "/opt/archivmail",
+    "response-message": "Deploy gestartet.",
+    "trigger-rule": {
+      "and": [
+        {
+          "match": {
+            "type": "payload-hmac-sha256",
+            "secret": "{{ getenv \"WEBHOOK_SECRET\" }}",
+            "parameter": {
+              "source": "header",
+              "name": "X-Hub-Signature-256"
+            }
+          }
+        },
+        {
+          "match": {
+            "type": "value",
+            "value": "refs/heads/main",
+            "parameter": {
+              "source": "payload",
+              "name": "ref"
+            }
+          }
+        }
+      ]
+    }
+  }
+]