feat(engine): merge cloudflare-migration — paradigm D engine, BYOK proxy, story persistence (#95)

Squash-merge the cloudflare-migration branch (7 commits by Kai ki) into
staging with conflict resolution, feature integration, and bug fixes.

Engine:
- Paradigm D: single-stream Writer replacing dual-phase Plan/Beats
- Delete Architect agent; story bible generated via Writer <plan> tag
- Modular prompt architecture (segments/registry/builder)
- StreamRouter for tagged stream splitting (<plan>/<story>/<choices>)

Infrastructure:
- Cloudflare Workers deployment (wrangler.jsonc, OpenNext adapter)
- D1 database schema + Drizzle ORM (scaffolded, not yet active)
- R2 storage helpers (scaffolded, not yet active)
- Story persistence API routes + client-side persistence

BYOK (Bring Your Own Key):
- /api/llm/user-proxy with SSRF-protected LLM proxy (+ requireUser auth)
- CORS-aware fetch in ai-client: auto-detect CORS failure, fallback to
  server proxy transparently via OpenAI SDK custom fetch
- BYO config support added to classify-freeform and vision routes
- SettingsModal CORS privacy notice (keys never logged/stored)

SSE streaming:
- engineClient.ts: fetchSSE helper for progressive scene events
- startSession/requestScene accept optional emit callback
- Fix SSE error event field name (error → message) in scene/start routes

i18n integration:
- Wire buildLanguageDirective into paradigm D's prompt builder
- Update corsNotice i18n keys (zh-CN/en/ja) with CORS proxy privacy text
- Preserve Session.language + LanguageSwitcher from i18n commit

Co-authored-by: Kai ki <155355644+zbf1009@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

lib/r2.ts

@@ -0,0 +1,104 @@
+import "server-only";
+
+import { getCloudflareContext } from "@opennextjs/cloudflare";
+
+/**
+ * R2 Storage封装 - 用户生成图片持久化
+ *
+ * Phase 1: 优先使用 Runware CDN URL（零额外存储成本），R2 key 作为可选持久化。
+ * Phase 2+: save 流程中可选地将场景图从 CDN fetch 后转存 R2，防 URL 过期。
+ */
+
+/**
+ * Build R2 object key for image storage.
+ *
+ * Pattern: {storyId}/{kind}/{id}.webp
+ *   - kind: "scene" | "portrait" | "style-ref"
+ *   - id: scene.id | character.name | "ref"
+ *
+ * Example: s_abc123/scene/sc_1.webp, s_abc123/portrait/李华.webp
+ */
+export function buildImageKey(
+  storyId: string,
+  kind: "scene" | "portrait" | "style-ref",
+  id: string,
+): string {
+  // Sanitize both storyId and id to avoid path traversal / key confusion
+  const safeStoryId = storyId.replace(/[^a-zA-Z0-9_一-龥-]/g, "_");
+  const safeId = id.replace(/[^a-zA-Z0-9_一-龥-]/g, "_");
+  return `${safeStoryId}/${kind}/${safeId}.webp`;
+}
+
+/**
+ * Upload image to R2 and return public URL.
+ *
+ * @param key R2 object key (use buildImageKey to generate)
+ * @param data Image data (Buffer or Uint8Array)
+ * @returns Public R2 URL (https://<public-domain>/<key>)
+ * @throws Error if R2 upload fails or binding unavailable
+ */
+export async function uploadImage(
+  key: string,
+  data: Buffer | Uint8Array,
+): Promise<string> {
+  try {
+    const { env } = getCloudflareContext();
+
+    if (!env.R2_BUCKET) {
+      throw new Error(
+        "R2_BUCKET binding not found. " +
+        "Ensure wrangler.jsonc has r2_buckets configured and you're running via wrangler."
+      );
+    }
+
+    // Upload to R2 with WebP content-type
+    await env.R2_BUCKET.put(key, data, {
+      httpMetadata: {
+        contentType: "image/webp",
+      },
+    });
+
+    // Return public URL (assumes custom domain or R2 public bucket configured)
+    // Phase 1: hardcode or read from env; Phase 2: configure in wrangler
+    const publicDomain = process.env.R2_PUBLIC_DOMAIN ?? "https://r2.infiplot.example"; // Placeholder
+    return `${publicDomain}/${key}`;
+  } catch (error) {
+    // Re-throw with context for caller to handle gracefully
+    throw new Error(
+      `R2 upload failed for key ${key}: ${error instanceof Error ? error.message : String(error)}`
+    );
+  }
+}
+
+/**
+ * Fetch image from URL and upload to R2 (for migrating Runware CDN → R2).
+ *
+ * @param url Source image URL (e.g. Runware CDN)
+ * @param key R2 object key
+ * @returns Public R2 URL, or null if fetch/upload fails (caller should fallback to original URL)
+ */
+export async function migrateImageToR2(
+  url: string,
+  key: string,
+): Promise<string | null> {
+  try {
+    // Fetch image from CDN
+    const res = await fetch(url);
+    if (!res.ok) {
+      console.warn(`[R2] Failed to fetch image from ${url}: HTTP ${res.status}`);
+      return null;
+    }
+
+    const data = new Uint8Array(await res.arrayBuffer());
+
+    // Upload to R2
+    return await uploadImage(key, data);
+  } catch (error) {
+    // Log but don't throw - caller should gracefully fallback to CDN URL
+    console.warn(
+      `[R2] Migration failed for ${url} → ${key}:`,
+      error instanceof Error ? error.message : error
+    );
+    return null;
+  }
+}