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/api/admin/events/[id].put.js

@@ -1,5 +1,4 @@
 import Event from '../../../models/event.js'
-import Tag from '../../../models/tag.js'
 import { connectDB } from '../../../utils/mongoose.js'
 import { requireAdmin } from '../../../utils/auth.js'
 
@@ -12,18 +11,7 @@ export default defineEventHandler(async (event) => {
 
     await connectDB()
 
-    // Validate tag slugs against Tag collection
-    if (body.tags && body.tags.length > 0) {
-      const foundTags = await Tag.find({ slug: { $in: body.tags } })
-      const foundSlugs = new Set(foundTags.map(t => t.slug))
-      const invalid = body.tags.filter(s => !foundSlugs.has(s))
-      if (invalid.length > 0) {
-        throw createError({
-          statusCode: 400,
-          statusMessage: `Unknown tag slugs: ${invalid.join(', ')}`
-        })
-      }
-    }
+    await validateTagSlugs(body.tags)
 
     const updateData = {
       ...body,