feat(14-01): add fetchMessages() to InboxProvider + both implementations

- Add InboxMessage type (subject + html/text body) and fetchMessages() to
  the InboxProvider interface, ImapProvider, and ExchangeInboxProvider
- IMAP: findBodyParts() walks the MIME tree for first text/html + text/plain
  parts, reusing the connect/lock/search/fetchAll skeleton; marks \Seen
- EWS: new getItemBodySoap() requests item:Body, extracts BodyType via the
  existing extractAttr/extractAll helpers, marks IsRead via markReadSoap
- Net-new spec coverage (imap.provider.spec.ts, exchange-inbox.provider.spec.ts)
  mocking ImapFlow and httpntlm.post (via require.cache stub, since httpntlm
  is loaded with a raw require() that vi.mock cannot intercept)
- fetchPdfAttachments untouched in both providers (D-02); full API suite
  (301 tests) + tsc --noEmit stay green

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/api/src/inbox/inbox-provider.interface.ts

@@ -1,7 +1,7 @@
-import { InboxAttachment, InboxConfig, InboxEmail } from './inbox.types';
+import { InboxAttachment, InboxConfig, InboxEmail, InboxMessage } from './inbox.types';
 
 // Re-export types for downstream consumers that import from this module
-export type { InboxAttachment, InboxConfig, InboxEmail };
+export type { InboxAttachment, InboxConfig, InboxEmail, InboxMessage };
 
 /**
  * Abstract inbox provider contract — implemented by ImapProvider and
@@ -29,4 +29,17 @@ export interface InboxProvider {
    * @returns       { success: true } on success, { success: false, message } on failure
    */
   testConnection(config: InboxConfig): Promise<{ success: boolean; message?: string }>;
+
+  /**
+   * Connects to the configured inbox, searches for unread emails (optionally
+   * filtered by sender), and returns each message's subject + HTML/text body.
+   *
+   * Additive method (D-02, Phase 14 inbox extraction) — the seam the
+   * EmailAlertAdapter (Plan 14-03) consumes. Does NOT affect
+   * fetchPdfAttachments; DKV is not switched to this method.
+   *
+   * @param config  Decrypted inbox connection parameters
+   * @returns       Array of messages with subject + body (empty array on error)
+   */
+  fetchMessages(config: InboxConfig): Promise<InboxMessage[]>;
 }