Add AI receipt parsing with provider API key settings

- Settings → AI: configure Anthropic or OpenAI provider with encrypted API key
- Sparkle button on each attachment in transaction drawer sends image/PDF to AI
- AI extracts merchant, amount, date, description, category hint
- "Apply to transaction" button patches the transaction with parsed fields
- Anthropic supports images and PDFs; OpenAI supports images only
- API key stored AES-256-GCM encrypted in users table (migration 0003)

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

frontend/src/api/settings.ts

@@ -0,0 +1,34 @@
+import { api } from "./client";
+
+export interface AiSettings {
+  provider: string | null;
+  has_api_key: boolean;
+}
+
+export interface ParsedReceipt {
+  merchant: string | null;
+  amount: number | null;
+  currency: string | null;
+  date: string | null;
+  description: string | null;
+  category: string | null;
+}
+
+export async function getAiSettings(): Promise<AiSettings> {
+  const { data } = await api.get("/settings/ai");
+  return data;
+}
+
+export async function saveAiSettings(provider: string, api_key: string): Promise<AiSettings> {
+  const { data } = await api.put("/settings/ai", { provider, api_key });
+  return data;
+}
+
+export async function clearAiSettings(): Promise<void> {
+  await api.delete("/settings/ai");
+}
+
+export async function parseReceipt(txnId: string, attachmentId: string): Promise<ParsedReceipt> {
+  const { data } = await api.post(`/transactions/${txnId}/attachments/${attachmentId}/parse`);
+  return data;
+}