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/classify-freeform/route.ts

@@ -1,7 +1,7 @@
 import { classifyFreeform } from "@infiplot/engine";
 import type { FreeformClassifyRequest } from "@infiplot/types";
 import { NextResponse } from "next/server";
-import { loadEngineConfig } from "@/lib/config";
+import { loadEngineConfig, buildByoEngineConfig } from "@/lib/config";
 import { requireUser } from "@/lib/supabase/guard";
 
 export const runtime = "nodejs";
@@ -25,11 +25,13 @@ export async function POST(req: Request) {
   }
 
   try {
-    const config = loadEngineConfig();
+    const official = loadEngineConfig();
+    const config = body.byo ? buildByoEngineConfig(body.byo, official) : official;
     const result = await classifyFreeform(config, body);
     return NextResponse.json(result);
   } catch (err) {
     const message = err instanceof Error ? err.message : "Unknown error";
-    return NextResponse.json({ error: message }, { status: 500 });
+    const status = message.includes("Invalid BYO") || message.includes("Missing BYO") ? 400 : 500;
+    return NextResponse.json({ error: message }, { status });
   }
 }