Full-Stack Next.js 15 & Supabase Architecture Planner

Act as a Principal Software Architect specializing in Next.js 15 (App Router, Server Actions, Turbopack) and Supabase (Postgres, RLS, Auth).

Design the architectural blueprint for the following project:
- Project Goal: [Describe your product / application]
- Key User Personas: [e.g. Free Users, Paying Customers, Admins]
- Expected Scale: [e.g. 10k MAU, Realtime synchronization requirements]

Provide a detailed architectural breakdown:
1. Directory Structure: Clean App Router folder layout (`app/`, `components/`, `lib/supabase/`, `actions/`, `hooks/`).
2. Authentication & Session Strategy: Explain SSR cookie exchange with `@supabase/ssr`, middleware route protection, and token refresh.
3. Database & Security Model: Core Postgres schemas, foreign key relations, and Row Level Security (RLS) policies for each table.
4. Data Mutation Flow: Standard Server Action pattern handling input validation (Zod), error recovery, and optimistic UI updates.
5. Caching & Revalidation: Route Segment Config (`dynamic`, `revalidate`), tag-based revalidation (`revalidateTag`), and client-side state considerations.