feat(ai): server-side chat history + assistant UX fixes

Server-side, per-user chat history (the thread now survives reload):
- new ai_chat_messages table + migration; getChatHistory/appendChatMessages/
  clearChatHistory; GET/POST/DELETE /api/admin/ai/history (ai.use-guarded);
  service + HTTP tests (isolation, ordering, clear, perm, validation).
- DashAssistant loads the thread on mount and persists each turn; "Vymazat"
  clears it. gcTime:0 so each mount re-reads the DB (no stale-cache resurrection);
  composer gated until history settles + seed flag flipped on submit (no
  seed/submit race clobbering a freshly-sent turn).

UX + correctness fixes from review/feedback:
- review cards moved OUT of the scrollable thread into a "Faktury k potvrzení"
  section so Uložit is always reachable; chat auto-scroll no longer fires on
  field edits.
- chat bubble text colour set on the Typography (GlobalStyles pins `p` to
  text.secondary, which was overriding the inherited bubble colour → unreadable).
- never clear input/staged PDFs on a failed submit (rollback + keep), so a
  budget error can't wipe staged invoices; roll back the optimistic user turn.
- stable attachment ids (no crypto.randomUUID — undefined over plain HTTP).
- editable Datum splatnosti + Popis fields; client-side extraction summary.

Security: DashProfile clipboard copy now uses an execCommand fallback (works
over plain HTTP) and only shows the success toast when the copy actually
succeeded — previously it falsely claimed to copy 2FA codes / the TOTP secret.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/services/ai.service.ts

@@ -104,6 +104,50 @@ export async function assertBudgetAvailable(): Promise<{
   return null;
 }
 
+// ── Chat history (server-side, permanent, per user) ───────────────────────
+export interface StoredChatMessage {
+  role: string;
+  content: string;
+  created_at: Date;
+}
+
+// Cap how many turns we read back / display. The full thread stays in the DB;
+// this only bounds a single GET payload (and, on the client, the model context).
+const HISTORY_LIMIT = 200;
+
+/** This user's chat thread, oldest → newest, capped at HISTORY_LIMIT. */
+export async function getChatHistory(
+  userId: number,
+): Promise<StoredChatMessage[]> {
+  const rows = await prisma.ai_chat_messages.findMany({
+    where: { user_id: userId },
+    orderBy: { id: "desc" },
+    take: HISTORY_LIMIT,
+    select: { role: true, content: true, created_at: true },
+  });
+  return rows.reverse();
+}
+
+/** Append turns to this user's thread (best-effort caller). */
+export async function appendChatMessages(
+  userId: number,
+  messages: { role: string; content: string }[],
+): Promise<void> {
+  if (messages.length === 0) return;
+  await prisma.ai_chat_messages.createMany({
+    data: messages.map((m) => ({
+      user_id: userId,
+      role: m.role,
+      content: m.content,
+    })),
+  });
+}
+
+/** Wipe this user's thread. */
+export async function clearChatHistory(userId: number): Promise<void> {
+  await prisma.ai_chat_messages.deleteMany({ where: { user_id: userId } });
+}
+
 export interface ChatMessage {
   role: "user" | "assistant";
   content: string;