feat(feed): optional per-feed sender-in-title toggle

Add a per-feed senderInTitle flag (domain FeedState.senderInTitle ↔
FeedConfig.sender_in_title). When set, the feed generator prefixes each
entry title with [Sender] (display name, falling back to the address).
Exposed as an admin edit-form checkbox and across the REST API
create/update/response schemas.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/types/index.ts

@@ -49,6 +49,8 @@ export interface FeedConfig {
   blocked_senders?: string[];
   language: string;
   author?: string;
+  // When true, entry titles in the feed output are rendered as `[Sender] Subject`.
+  sender_in_title?: boolean;
   created_at: number;
   updated_at?: number;
   expires_at?: number; // Unix timestamp ms — present when a TTL is configured