refactor: extract escapeRegex and validateTagSlugs server utils

Deduplicate tag validation and regex escaping into shared auto-imported
utils. Add tag validation to wiki patch/batch-tag routes. Remove
duplicate tags field from event schema.

server/utils/escapeRegex.js

@@ -0,0 +1,3 @@
+export function escapeRegex(str) {
+  return str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')
+}

server/utils/validateTagSlugs.js

@@ -0,0 +1,14 @@
+import Tag from '../models/tag.js'
+
+export async function validateTagSlugs(slugs) {
+  if (!slugs?.length) return
+  const foundTags = await Tag.find({ slug: { $in: slugs } })
+  const foundSlugs = new Set(foundTags.map(t => t.slug))
+  const invalid = slugs.filter(s => !foundSlugs.has(s))
+  if (invalid.length > 0) {
+    throw createError({
+      statusCode: 400,
+      statusMessage: `Unknown tag slugs: ${invalid.join(', ')}`
+    })
+  }
+}