feat(dkv): Exchange subfolder support via EWS FindFolder

Custom folder names (e.g. "DKV" or "INBOX/DKV") now resolved by calling
EWS FindFolder deep-search under inbox. Well-known names still map to
DistinguishedFolderId directly. Falls back to inbox with a warning log
when the subfolder cannot be found.

IMAP already supported subfolder paths natively via ImapFlow.

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

apps/api/src/dkv/providers/exchange-inbox.provider.ts

@@ -23,7 +23,8 @@ function soapEnvelope(body: string): string {
 </soap:Envelope>`;
 }
 
-function findItemSoap(folderId: string, maxResults: number, senderFilter?: string): string {
+/** folderElement: either <t:DistinguishedFolderId Id="inbox"/> or <t:FolderId Id="AAA..."/> */
+function findItemSoap(folderElement: string, maxResults: number, senderFilter?: string): string {
   const isReadFilter = `<t:IsEqualTo>
         <t:FieldURI FieldURI="message:IsRead"/>
         <t:FieldURIOrConstant><t:Constant Value="false"/></t:FieldURIOrConstant>
@@ -56,7 +57,7 @@ function findItemSoap(folderId: string, maxResults: number, senderFilter?: strin
       <m:IndexedPageItemView MaxEntriesReturned="${maxResults}" Offset="0" BasePoint="Beginning"/>
       ${restriction}
       <m:ParentFolderIds>
-        <t:DistinguishedFolderId Id="${escapeXml(folderId)}"/>
+        ${folderElement}
       </m:ParentFolderIds>
     </m:FindItem>`);
 }
@@ -157,24 +158,46 @@ function extractAttrs(xml: string, tag: string, attr: string): string[] {
   return results;
 }
 
-/** Map configured folder name to EWS DistinguishedFolderId. */
-function resolveDistinguishedFolder(folder?: string): string {
+const DISTINGUISHED_FOLDER_MAP: Record<string, string> = {
+  inbox: 'inbox',
+  posteingang: 'inbox',
+  deleteditems: 'deleteditems',
+  gelöschteelemente: 'deleteditems',
+  trash: 'deleteditems',
+  sentitems: 'sentitems',
+  gesendet: 'sentitems',
+  drafts: 'drafts',
+  entwürfe: 'drafts',
+  junk: 'junkemail',
+  junkemail: 'junkemail',
+  spam: 'junkemail',
+};
+
+/** Returns the DistinguishedFolderId if folder is a well-known name, else null (→ needs FindFolder). */
+function resolveDistinguishedFolder(folder?: string): string | null {
   const name = (folder ?? 'INBOX').toLowerCase().replace(/[\s_-]/g, '');
-  const map: Record<string, string> = {
-    inbox: 'inbox',
-    posteingang: 'inbox',
-    deleteditems: 'deleteditems',
-    gelöschteelemente: 'deleteditems',
-    trash: 'deleteditems',
-    sentitems: 'sentitems',
-    gesendet: 'sentitems',
-    drafts: 'drafts',
-    entwürfe: 'drafts',
-    junk: 'junkemail',
-    junkemail: 'junkemail',
-    spam: 'junkemail',
-  };
-  return map[name] ?? 'inbox';
+  return DISTINGUISHED_FOLDER_MAP[name] ?? null;
+}
+
+/** Build FindFolder SOAP to resolve a subfolder by display name under a parent DistinguishedFolderId. */
+function findFolderSoap(parentDistinguishedId: string, displayName: string): string {
+  return soapEnvelope(`
+    <m:FindFolder Traversal="Deep">
+      <m:FolderShape>
+        <t:BaseShape>IdOnly</t:BaseShape>
+      </m:FolderShape>
+      <m:Restriction>
+        <t:IsEqualTo>
+          <t:FieldURI FieldURI="folder:DisplayName"/>
+          <t:FieldURIOrConstant>
+            <t:Constant Value="${escapeXml(displayName)}"/>
+          </t:FieldURIOrConstant>
+        </t:IsEqualTo>
+      </m:Restriction>
+      <m:ParentFolderIds>
+        <t:DistinguishedFolderId Id="${escapeXml(parentDistinguishedId)}"/>
+      </m:ParentFolderIds>
+    </m:FindFolder>`);
 }
 
 // ─── NTLM HTTP helper ────────────────────────────────────────────────────────
@@ -227,8 +250,8 @@ export class ExchangeInboxProvider implements InboxProvider {
 
   async testConnection(config: InboxConfig): Promise<{ success: boolean; message?: string }> {
     try {
-      const folder = resolveDistinguishedFolder(config.folder);
-      const soap = findItemSoap(folder, 1);
+      const folderElement = await this.resolveFolderElement(config);
+      const soap = findItemSoap(folderElement, 1);
       const res = await this.ewsPost(config, soap, 'FindItem');
 
       if (res.statusCode === 401) {
@@ -251,11 +274,34 @@ export class ExchangeInboxProvider implements InboxProvider {
 
   // ─── Private ───────────────────────────────────────────────────────────────
 
+  /** Resolve folder config to a EWS ParentFolderIds XML element.
+   *  Well-known names → <t:DistinguishedFolderId>.
+   *  Custom names (e.g. "DKV" or "INBOX/DKV") → FindFolder deep search under inbox → <t:FolderId>.
+   */
+  private async resolveFolderElement(config: InboxConfig): Promise<string> {
+    const folderCfg = config.folder ?? 'INBOX';
+    // Strip leading "INBOX/" prefix — EWS FindFolder searches deep, name alone suffices
+    const displayName = folderCfg.replace(/^INBOX\//i, '').trim();
+    const distinguished = resolveDistinguishedFolder(displayName);
+    if (distinguished) {
+      return `<t:DistinguishedFolderId Id="${escapeXml(distinguished)}"/>`;
+    }
+    // Custom subfolder: resolve via FindFolder
+    const ffSoap = findFolderSoap('inbox', displayName);
+    const ffRes = await this.ewsPost(config, ffSoap, 'FindFolder');
+    const folderId = extractAttr(ffRes.body, 't:FolderId', 'Id');
+    if (!folderId) {
+      this.logger.warn(`EWS FindFolder: subfolder "${displayName}" not found — falling back to inbox`);
+      return `<t:DistinguishedFolderId Id="inbox"/>`;
+    }
+    return `<t:FolderId Id="${escapeXml(folderId)}"/>`;
+  }
+
   private async fetchViaEws(config: InboxConfig): Promise<InboxEmail[]> {
-    const folder = resolveDistinguishedFolder(config.folder);
+    const folderElement = await this.resolveFolderElement(config);
 
     // 1. FindItem — get IDs of emails with attachments
-    const findSoap = findItemSoap(folder, 50, config.senderFilter);
+    const findSoap = findItemSoap(folderElement, 50, config.senderFilter);
     const findRes = await this.ewsPost(config, findSoap, 'FindItem');
     if (findRes.statusCode !== 200) {
       this.logger.warn(`EWS FindItem returned HTTP ${findRes.statusCode}`);