fix: batch 5 — input validation tightening + health check endpoint

- z.any() replaced with z.record(z.string()) on webhook headers
- availabilityJson typed with z.array(z.object({ start, end }))
- Frontend webhook headers converted from array to Record before API call
- Docker HEALTHCHECK added to Dockerfile (health endpoint already existed)

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

src/app/(admin)/admin/settings/webhooks/page.tsx

@@ -210,11 +210,16 @@ export default function WebhooksPage() {
       return
     }
 
+    const filteredHeaders = formData.headers.filter((h) => h.key)
+    const headersRecord = filteredHeaders.length > 0
+      ? Object.fromEntries(filteredHeaders.map((h) => [h.key, h.value]))
+      : undefined
+
     const payload = {
       name: formData.name,
       url: formData.url,
       events: formData.events,
-      headers: formData.headers.filter((h) => h.key) as Record<string, string>[] | undefined,
+      headers: headersRecord,
       maxRetries: formData.maxRetries,
     }