feat: turbostarter boilerplate

Production-ready Next.js boilerplate with:
- Runtime env validation (fail-fast on missing vars)
- Feature-gated config (S3, Stripe, email, OAuth)
- Docker + Coolify deployment pipeline
- PostgreSQL + pgvector, MinIO S3, Better Auth
- TypeScript strict mode (no ignoreBuildErrors)
- i18n (en/es), AI modules, billing, monitoring

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.context/turbostarter-framework-context/sections/ai/docs/auth.md

@@ -0,0 +1,41 @@
+---
+title: Authentication
+description: Learn about the authentication flow in TurboStarter AI.
+url: /ai/docs/auth
+---
+
+# Authentication
+
+TurboStarter AI implements a streamlined authentication approach powered by [Better Auth](https://www.better-auth.com/). Since the primary focus is showcasing AI capabilities, we've kept the initial authentication simple, allowing you to quickly integrate and experiment with AI features.
+
+## Anonymous sessions
+
+When someone first visits the AI application, an **anonymous session** is automatically created. This establishes a unique user identity without requiring login credentials.
+
+These anonymous sessions serve two critical purposes:
+
+1. **Persistence:** links data like chat history or generated content to specific users in your database
+2. **Usage control:** enables tracking for rate limiting and the credits system, ensuring fair AI resource usage even for anonymous visitors
+
+## Extending authentication
+
+While the default anonymous setup provides a frictionless initial experience, TurboStarter is built for growth. The authentication logic uses Better Auth in the shared `packages/auth` package, ensuring consistency between web and mobile applications.
+
+When your project needs more sophisticated authentication features like:
+
+* Email/Password login
+* Magic links
+* Social logins (OAuth)
+* Multi-factor authentication
+
+You can easily integrate these by leveraging the comprehensive authentication system in the [TurboStarter Core kit](/docs/web). The underlying structure is already in place, making this transition straightforward.
+
+For detailed implementation guides, check out the core documentation:
+
+<Cards>
+  <Card title="Web authentication" href="/docs/web/auth/overview" description="Explore the full authentication capabilities for the web application." />
+
+  <Card title="Mobile authentication" href="/docs/mobile/auth/overview" description="Learn how authentication works within the mobile application." />
+</Cards>
+
+By starting with anonymous sessions, the AI kit lets you focus on building compelling AI features first, while providing a clear path to implement advanced user management and security as your application evolves.