feat: refactor to FastAPI architecture v2.0

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

app/static/js/api.js

@@ -0,0 +1,61 @@
+/**
+ * API fetch wrapper for Scenar Creator.
+ */
+
+const API = {
+    async post(url, body, isJson = true) {
+        const opts = { method: 'POST' };
+        if (isJson) {
+            opts.headers = { 'Content-Type': 'application/json' };
+            opts.body = JSON.stringify(body);
+        } else {
+            opts.body = body; // FormData
+        }
+        const res = await fetch(url, opts);
+        return res;
+    },
+
+    async postJson(url, body) {
+        const res = await this.post(url, body, true);
+        if (!res.ok) {
+            const err = await res.json().catch(() => ({ detail: res.statusText }));
+            throw new Error(err.detail || 'API error');
+        }
+        return res.json();
+    },
+
+    async postBlob(url, body) {
+        const res = await this.post(url, body, true);
+        if (!res.ok) {
+            const err = await res.json().catch(() => ({ detail: res.statusText }));
+            throw new Error(err.detail || 'API error');
+        }
+        return res.blob();
+    },
+
+    async postFormData(url, formData) {
+        const res = await this.post(url, formData, false);
+        if (!res.ok) {
+            const err = await res.json().catch(() => ({ detail: res.statusText }));
+            throw new Error(err.detail || 'API error');
+        }
+        return res.json();
+    },
+
+    async get(url) {
+        const res = await fetch(url);
+        if (!res.ok) throw new Error(res.statusText);
+        return res.json();
+    },
+
+    downloadBlob(blob, filename) {
+        const url = URL.createObjectURL(blob);
+        const a = document.createElement('a');
+        a.href = url;
+        a.download = filename;
+        document.body.appendChild(a);
+        a.click();
+        document.body.removeChild(a);
+        URL.revokeObjectURL(url);
+    }
+};