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/domain/feed-keys.ts

@@ -13,6 +13,9 @@ export const feedKeys = {
   config: (feedId: string): string => `feed:${feedId}:config`,
   metadata: (feedId: string): string => `feed:${feedId}:metadata`,
 
+  /** Secondary index: inbound mailbox local part → feed id (resolved at reception). */
+  inbound: (mailboxId: string): string => `inbound:${mailboxId}`,
+
   /** Prefix covering every key owned by a feed (config, metadata, emails). */
   feedPrefix: (feedId: string): string => `feed:${feedId}:`,