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>

app/api/stories/featured/route.ts

@@ -0,0 +1,48 @@
+import { NextResponse } from "next/server";
+import { getDb } from "@/lib/db/client";
+import { FeaturedRepository } from "@/lib/db/repositories/featuredRepo";
+
+export const runtime = "nodejs";
+
+/**
+ * GET /api/stories/featured?gender=male
+ *
+ * List active featured stories for homepage display.
+ * Fallback: D1 query fails → return empty array (homepage shows no cards, gracefully degrades).
+ *
+ * Query Params:
+ *   gender: "male" | "female" (required)
+ *
+ * Response: { stories: FeaturedStory[] }
+ * Errors: 400 (invalid gender), 500 (should not reach user - caught and degraded)
+ */
+export async function GET(req: Request) {
+  const { searchParams } = new URL(req.url);
+  const genderParam = searchParams.get("gender");
+
+  // Validate gender
+  if (!genderParam || !["male", "female"].includes(genderParam)) {
+    return NextResponse.json(
+      { error: "gender query parameter must be 'male' or 'female'" },
+      { status: 400 },
+    );
+  }
+
+  const gender = genderParam as "male" | "female";
+
+  try {
+    const db = getDb();
+    const repo = new FeaturedRepository(db);
+
+    const stories = await repo.listByGender(gender);
+
+    return NextResponse.json({ stories });
+  } catch (err) {
+    // D1 unavailable or query failed - degrade to empty array
+    // (homepage will show no cards but remain functional)
+    const message = err instanceof Error ? err.message : "Unknown error";
+    console.error("[stories/featured] D1 query failed, returning empty array:", message);
+
+    return NextResponse.json({ stories: [] });
+  }
+}