docs: add constitution, spec, plan, and data model for InteriorScan MVP

- Constitution: self-hosted first, FPS-grade immersion, mobile-first capture, API-first, open formats
- Spec: 6 user stories (upload & process, share & embed, dashboard, API, measurements, billing)
- Plan: 6 phases, 50 tasks, Phase 1-3 = MVP core
- Data model: Drizzle schema with projects, photos, walkthroughs, processing_jobs, subscriptions

README.md

@@ -1,3 +1,59 @@
-# interiorscan
+# InteriorScan
 
-Interior photogrammetry SaaS - Doom-map style 3D walkthroughs for real estate
+**Turn apartment photos into navigable 3D walkthroughs.**
+
+InteriorScan is a self-hosted SaaS platform for interior photogrammetry targeting real estate. Upload 30-100 photos of a property, the system reconstructs a 3D mesh (COLMAP + OpenMVS), and visitors navigate the space in first-person — like a Doom map of a real home.
+
+## Why?
+
+Physical property visits are time-consuming and often unnecessary. InteriorScan lets potential buyers walk through a reconstructed interior from their browser, filtering out properties that don't fit before scheduling a visit.
+
+## Features
+
+- 📸 **Photo-first capture** — iPhone/Android photos as baseline, LiDAR optional
+- 🎮 **FPS navigation** — WASD + mouse (desktop), tap-to-walk (mobile)
+- 🔗 **Share & embed** — public links + oEmbed for listing pages
+- 📐 **Measurements** — room dimensions overlaid on walkthrough
+- 🗺️ **Floor plans** — DXF/PDF export from reconstructed geometry
+- 🔌 **API-first** — every feature has REST API before UI
+- 🏠 **Self-hosted** — PostgreSQL + Valkey + GPU worker, no vendor lock-in
+
+## Stack
+
+| Layer | Technology |
+|-------|-----------|
+| Framework | Next.js 16 (App Router) |
+| Monorepo | Turborepo + pnpm |
+| Database | PostgreSQL + Drizzle ORM |
+| Cache/Queue | Valkey |
+| 3D Viewer | three.js |
+| Processing | COLMAP + OpenMVS |
+| Auth | better-auth |
+| Billing | Stripe |
+| Lint | Biome 2.x |
+| Testing | Vitest v4 + Playwright |
+
+## Quick Start
+
+```bash
+pnpm install
+pnpm dev
+```
+
+## Project Structure
+
+```
+interiorscan/
+├── apps/web/              # Next.js 16 App Router
+├── apps/processor/        # GPU processing worker
+├── packages/auth/         # better-auth + RBAC
+├── packages/db/           # Drizzle schema + migrations
+├── packages/viewer/       # three.js FPS walkthrough viewer
+├── packages/ui/           # shadcn/ui v4 + Tailwind tokens
+├── packages/utils/        # Zod, neverthrow, error catalog
+└── .specify/              # Constitution, specs, plans
+```
+
+## License
+
+Apache-2.0