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>

package.json

@@ -16,29 +16,35 @@
     "lint": "next lint",
     "typecheck": "tsc --noEmit",
     "enrich:firstacts": "node scripts/enrich-firstacts-stepfun.mjs",
-    "build:cf": "opennextjs-cloudflare build",
+    "build:cf": "cross-env BUILD_STANDALONE=true next build --webpack && opennextjs-cloudflare build --skipNextBuild",
     "preview:cf": "opennextjs-cloudflare preview",
     "deploy:cf": "opennextjs-cloudflare deploy"
   },
   "dependencies": {
     "@supabase/ssr": "^0.12",
     "@supabase/supabase-js": "^2.108",
+    "drizzle-orm": "^0.45.2",
     "jsonrepair": "^3.14.0",
     "jszip": "^3.10.1",
     "next": "^16.0.0",
     "openai": "^6.42.0",
     "react": "^19.0.0",
-    "react-dom": "^19.0.0"
+    "react-dom": "^19.0.0",
+    "server-only": "^0.0.1"
   },
   "devDependencies": {
+    "@cloudflare/workers-types": "^4.20260607.1",
     "@opennextjs/cloudflare": "^1.19.11",
     "@types/node": "^22.9.0",
     "@types/react": "^19.0.0",
     "@types/react-dom": "^19.0.0",
     "autoprefixer": "^10.4.20",
+    "cross-env": "^10.1.0",
+    "drizzle-kit": "^0.31.10",
     "postcss": "^8.4.49",
     "sharp": "^0.33.5",
     "tailwindcss": "^3.4.15",
+    "tsx": "^4.22.4",
     "typescript": "^5.6.3",
     "wrangler": "^4.96.0"
   }