feat: add resend creation email button and fix Handlebars template processing

- Fixed email template processing to properly handle Handlebars conditionals
  - Installed handlebars package
  - Updated processTemplate to use Handlebars.compile()
  - Now {{#if welcome_message}} blocks work correctly

- Added "Resend Creation Email" button to event details page
  - New endpoint: POST /api/admin/events/:id/resend-email
  - Button appears below "Reset Gallery Password"
  - Shows mail icon and includes success/error notifications
  - For security, resent emails show "(Aus Sicherheitsgründen nicht angezeigt)"
    instead of the actual password

- Added translations:
  - EN: "Resend Creation Email", success/error messages
  - DE: "Erstellungs-E-Mail erneut senden", success/error messages

This fixes the issue where Handlebars syntax was showing in emails and provides
a convenient way to resend the initial gallery creation email to hosts.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

backend/package-lock.json

@@ -19,6 +19,7 @@
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         "express-validator": "^7.0.1",
         "form-data": "^4.0.3",
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         "i18next-browser-languagedetector": "^8.2.0",
@@ -3995,6 +3996,27 @@
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       "license": "MIT"
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+      "license": "MIT",
+      "dependencies": {
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+        "neo-async": "^2.6.2",
+        "source-map": "^0.6.1",
+        "wordwrap": "^1.0.0"
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+        "handlebars": "bin/handlebars"
+      },
+      "engines": {
+        "node": ">=0.4.7"
+      },
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@@ -5985,6 +6007,12 @@
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       }
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+    "node_modules/neo-async": {
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+      "resolved": "https://registry.npmjs.org/neo-async/-/neo-async-2.6.2.tgz",
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-      "dev": true,
       "license": "BSD-3-Clause",
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@@ -8161,6 +8188,19 @@
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       "license": "MIT"
     },
+    "node_modules/uglify-js": {
+      "version": "3.19.3",
+      "resolved": "https://registry.npmjs.org/uglify-js/-/uglify-js-3.19.3.tgz",
+      "integrity": "sha512-v3Xu+yuwBXisp6QYTcH4UbH+xYJXqnq2m/LtQVWKWzYc1iehYnLixoQDN9FH6/j9/oybfd6W9Ghwkl8+UMKTKQ==",
+      "license": "BSD-2-Clause",
+      "optional": true,
+      "bin": {
+        "uglifyjs": "bin/uglifyjs"
+      },
+      "engines": {
+        "node": ">=0.8.0"
+      }
+    },
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       "version": "2.0.5",
       "resolved": "https://registry.npmjs.org/undefsafe/-/undefsafe-2.0.5.tgz",
@@ -8412,6 +8452,12 @@
         "node": ">=0.10.0"
       }
     },
+    "node_modules/wordwrap": {
+      "version": "1.0.0",
+      "resolved": "https://registry.npmjs.org/wordwrap/-/wordwrap-1.0.0.tgz",
+      "integrity": "sha512-gvVzJFlPycKc5dZN4yPkP8w7Dc37BtP1yczEneOb4uq34pXZcvrtRTmWV8W+Ume+XCxKgbjM+nevkyFPMybd4Q==",
+      "license": "MIT"
+    },
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