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>

cloudflare-env.d.ts

@@ -0,0 +1,16 @@
+/**
+ * Extend the global CloudflareEnv interface (declared by @opennextjs/cloudflare)
+ * with infiplot's D1/R2/KV bindings.
+ * See wrangler.jsonc for the binding configuration.
+ */
+
+interface CloudflareEnv {
+  // D1 Database binding (wrangler.jsonc: d1_databases)
+  DB: D1Database;
+
+  // R2 Bucket binding (wrangler.jsonc: r2_buckets)
+  R2_BUCKET: R2Bucket;
+
+  // KV Namespace binding (wrangler.jsonc: kv_namespaces)
+  KV: KVNamespace;
+}