Add optional info field to offers

- Add info field to Offer and CreateOfferRequest types
- Validate info field: optional, max 128 characters
- Include info field in all public API responses
- Update README with info field documentation

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

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

src/storage/types.ts

@@ -10,6 +10,7 @@ export interface Offer {
   expiresAt: number;
   lastSeen: number;
   secret?: string;
+  info?: string;
   answererPeerId?: string;
   answerSdp?: string;
   answeredAt?: number;
@@ -46,6 +47,7 @@ export interface CreateOfferRequest {
   topics: string[];
   expiresAt: number;
   secret?: string;
+  info?: string;
 }
 
 /**