feat: decouple read FeedId from inbound MailboxId

Separate the two feed identities so the public read URL never reveals the
inbound address and vice-versa:

- FeedId becomes an opaque high-entropy token (read id + KV key); MailboxId
  (noun.noun.NN) owns the inbound address and the untrusted-input boundary
  via MailboxId.parse. They map only through the inbound:<mailbox> secondary
  index, resolved solely at reception.
- inbound index lifecycle is owned by FeedRepository: written by save/saveConfig,
  dropped by removeFromList(Bulk) — symmetric, never mirrored by hand (removes the
  manual delete in feed-service + the cron loop, and a silent empty-catch).
- Feed.mailboxId exposes a MailboxId VO (symmetry with Feed.id); the
  mailbox@domain shape lives on MailboxId.emailAddress(domain).
- Distinguish mailbox_unknown (no feed claims the address) from feed_not_found
  (dangling index) for observability; both forwardable, both 404.
- Drop the redundant EmailParser.extractMailbox pass-through so MailboxId.parse
  is the single parse boundary.

Docs (README/INSTALL/CLAUDE.md/landing) and tests updated; 439 tests green,
tsc clean, build dry-run OK.

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

src/types/index.ts

@@ -42,6 +42,9 @@ export interface EmailData {
 export interface FeedConfig {
   title: string;
   description?: string;
+  // Inbound mailbox local part (noun.noun.NN): the feed's email address is
+  // `mailbox_id@domain`. Decoupled from the feed's id (the opaque read id).
+  mailbox_id: string;
   allowed_senders?: string[];
   blocked_senders?: string[];
   language: string;
@@ -82,6 +85,7 @@ export interface FeedListItem {
   id: string;
   title: string;
   description?: string;
+  mailbox_id: string; // Cached inbound address local part (admin/API display)
   expires_at?: number; // Cached from FeedConfig to avoid per-feed KV reads
 }