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 <[email protected]>

src/infrastructure/feed-mapper.ts

@@ -16,6 +16,7 @@ export function fromConfigDTO(dto: FeedConfig): FeedState {
     title: dto.title,
     description: dto.description,
     language: dto.language,
+    mailboxId: dto.mailbox_id,
     author: dto.author,
     allowedSenders: dto.allowed_senders ?? [],
     blockedSenders: dto.blocked_senders ?? [],
@@ -31,6 +32,7 @@ export function toConfigDTO(state: FeedState): FeedConfig {
     title: state.title,
     description: state.description,
     language: state.language,
+    mailbox_id: state.mailboxId,
     author: state.author,
     allowed_senders: state.allowedSenders,
     blocked_senders: state.blockedSenders,
@@ -46,6 +48,7 @@ export function toListItemDTO(id: FeedId, state: FeedState): FeedListItem {
     id: id.value,
     title: state.title,
     description: state.description,
+    mailbox_id: state.mailboxId,
     expires_at: state.expiresAt,
   };
 }