fix: preserve value types in extraFields (no string coercion)

- DocumentMeta.extraFields type -> Record<string, unknown>
- API routes pass extraFields through without JSON.stringify coercion
- CustomFieldBadges auto-styles by value type/content (number->purple, boolean->green/red, etc.)
- DocForm help text: generic (no hardcoded field examples)

packages/types/src/index.ts

@@ -16,7 +16,8 @@ export interface DocumentMeta {
   updatedAt: string; // ISO
   // Dynamic custom frontmatter fields (e.g. CTF: event, challenge, category,
   // difficulty, points). Content creators add arbitrary key-value pairs.
-  extraFields?: Record<string, string>;
+  // Values can be strings, numbers, booleans, arrays, or objects.
+  extraFields?: Record<string, unknown>;
 }
 
 export interface Document extends DocumentMeta {