feat(PROJ-30): Xapian → Manticore Search Migration

- internal/index/manticore.go: ManticoreTenantManager + manticoreIndex (RT-Indizes, CGO-frei)
- internal/index/index.go: TenantIndexer Interface (Xapian + Manticore)
- internal/index/tenant_worker.go: mgr-Typ auf TenantIndexer Interface
- internal/api/server.go: idxMgr auf TenantIndexer Interface
- config/config.go: IndexConfig.ManticoreDSN Feld
- cmd/archivmail/cmd_reindex.go: reindex Subkommando
- cmd/archivmail/main.go: Manticore-Branch + reindex Case
- go.mod: github.com/go-sql-driver/mysql v1.8.1
- update.sh: Manticore auto-install, CGO_ENABLED=0, config.yml migration, auto-reindex

fix(IMAP): TCP-Deadline-Wrapper für steckengebliebene Imports
fix(auth): Email-Claim in JWT für User-Isolation
fix(search): User-Isolation via sess.Email (fail-safe)
fix(ui): Admin-Login Auth-Cache, Logout-Redirect, IMAP-Polling-Resilienz

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

src/app/page.tsx

@@ -3,7 +3,7 @@
 import { useState, useEffect } from "react";
 import { useRouter } from "next/navigation";
 import { login } from "@/lib/api";
-import { getCachedUser } from "@/lib/auth-cache";
+import { getCachedUser, setCachedUser, clearAuthCache } from "@/lib/auth-cache";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { Card, CardContent, CardHeader, CardTitle } from "@/components/ui/card";
@@ -32,12 +32,14 @@ export default function LoginPage() {
     setLoading(true);
 
     try {
+      clearAuthCache();
       const res = await login(username, password);
       const role = res?.user?.role ?? "";
       if (ADMIN_ROLES.includes(role)) {
-        setError("Admins und Auditoren bitte über /admin anmelden.");
+        setError("ADMIN_REDIRECT");
         return;
       }
+      setCachedUser({ username: res.user.username, email: res.user.email, role });
       router.push("/search");
     } catch {
       setError("Anmeldung fehlgeschlagen. Bitte Zugangsdaten pruefen.");
@@ -83,11 +85,20 @@ export default function LoginPage() {
                 aria-label="Passwort"
               />
             </div>
-            {error && (
+            {error && error !== "ADMIN_REDIRECT" && (
               <p className="text-sm text-destructive" role="alert">
                 {error}
               </p>
             )}
+            {error === "ADMIN_REDIRECT" && (
+              <p className="text-sm text-destructive" role="alert">
+                Admins und Auditoren bitte{" "}
+                <a href="/admin/login" className="underline font-medium">
+                  hier anmelden
+                </a>
+                .
+              </p>
+            )}
             <Button type="submit" className="w-full" disabled={loading}>
               {loading ? "Anmelden..." : "Anmelden"}
             </Button>