feat: replace fixed 50-entry cap with size-based feed trimming

Emails are now trimmed from the oldest end when total serialised size
exceeds FEED_MAX_SIZE_BYTES (default 512 KB). Each EmailMetadata entry
stores its size so future trims are computed without re-reading KV.
Adds FEED_MAX_SIZE_BYTES, PROXY_TRUSTED_IPS and PROXY_AUTH_SECRET to Env.

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

src/types/index.ts

@@ -3,6 +3,9 @@ export interface Env {
   EMAIL_STORAGE: KVNamespace;
   ADMIN_PASSWORD: string;
   DOMAIN: string;
+  FEED_MAX_SIZE_BYTES?: string;
+  PROXY_TRUSTED_IPS?: string;
+  PROXY_AUTH_SECRET?: string;
 }
 
 // Email interface for stored emails
@@ -37,6 +40,7 @@ export interface EmailMetadata {
   key: string;
   subject: string;
   receivedAt: number;
+  size?: number;
 }
 
 // Feed list interface