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/llm/user-proxy/route.ts

@@ -0,0 +1,43 @@
+import { proxyLLM, type ProxyLLMParams } from "@/lib/byoProxy";
+import { NextResponse } from "next/server";
+import { requireUser } from "@/lib/supabase/guard";
+
+export const runtime = "nodejs";
+
+export async function POST(req: Request): Promise<Response> {
+  const auth = await requireUser();
+  if (auth instanceof NextResponse) return auth;
+  let parsed: Partial<ProxyLLMParams>;
+  try {
+    parsed = await req.json();
+  } catch {
+    return NextResponse.json({ error: "Invalid JSON body" }, { status: 400 });
+  }
+
+  // Validate required fields
+  const { provider, apiKey, baseUrl, body } = parsed;
+  if (!provider || !apiKey || !baseUrl || !body) {
+    return NextResponse.json(
+      { error: "Missing required fields: provider, apiKey, baseUrl, body" },
+      { status: 400 },
+    );
+  }
+
+  // Validate provider
+  if (!["openai", "claude", "gemini"].includes(provider)) {
+    return NextResponse.json(
+      { error: `Unsupported provider: ${provider}` },
+      { status: 400 },
+    );
+  }
+
+  // Forward to proxy core
+  return proxyLLM({
+    provider: provider as "openai" | "claude" | "gemini",
+    apiKey,
+    baseUrl,
+    body,
+    model: parsed.model,
+    stream: parsed.stream,
+  });
+}