docs: add ADR for Kamal deployment, remove old VPS guide

- Add docs/decisions/001-kamal-deployment.md documenting the decision
  to use Kamal instead of PM2+nginx for production deployment
- Remove docs/vps-deployment.md (superseded by Kamal approach)

Key decision drivers:
- Multi-app hosting support for future applications
- Built-in HA capability with multiple servers
- Docker-based consistency between environments
- Zero-downtime deployments via Traefik

Author: zainfathoni

docs/decisions/001-kamal-deployment.md

@@ -0,0 +1,120 @@
+# ADR 001: Use Kamal for Production Deployment
+
+## Status
+
+Accepted
+
+## Date
+
+2026-01-01
+
+## Context
+
+The production site has been down since PlanetScale (MySQL) shutdown. We need to
+deploy the application with the migrated SQLite database. The original plan was
+to use a traditional VPS deployment with PM2 + nginx (documented in
+`docs/vps-deployment.md`).
+
+However, we have additional requirements:
+
+1. **Multi-app hosting**: Want to host multiple applications on the same VPS
+2. **High availability**: Future-proof for HA setup with multiple servers
+3. **Consistent environments**: Avoid "works on my machine" issues
+4. **Zero-downtime deployments**: Rolling deploys without manual intervention
+
+## Decision
+
+Use **Kamal** (formerly MRSK) for deployment instead of bare PM2 + nginx.
+
+Kamal is a Docker-based deployment tool by 37signals (Basecamp/Hey.com) that:
+
+- Orchestrates Docker containers across servers
+- Manages Traefik as reverse proxy with automatic SSL
+- Provides zero-downtime rolling deployments
+- Supports multi-app hosting natively
+- Makes HA setup straightforward (just add more servers)
+
+## Alternatives Considered
+
+### 1. PM2 + nginx (Original Plan)
+
+**Pros:**
+
+- Simple, well-understood
+- Lower resource overhead (no Docker)
+- Documented in `docs/vps-deployment.md`
+
+**Cons:**
+
+- Manual nginx configuration per app
+- Complex HA setup
+- Environment drift between dev and prod
+- Manual SSL certificate management
+
+### 2. Docker Compose
+
+**Pros:**
+
+- Container consistency
+- Simple for single-server setups
+
+**Cons:**
+
+- No built-in deployment orchestration
+- Manual SSL setup
+- Limited multi-server support
+
+### 3. Kubernetes
+
+**Pros:**
+
+- Industry standard for container orchestration
+- Excellent HA and scaling
+
+**Cons:**
+
+- Massive overkill for this project size
+- Significant operational complexity
+- Higher resource requirements
+
+## Consequences
+
+### Positive
+
+- **Future-proof**: Easy path to HA by adding servers to config
+- **Consistent**: Same Docker image runs everywhere
+- **Simple SSL**: Traefik handles Let's Encrypt automatically
+- **Multi-app ready**: Each app gets its own `config/deploy.yml`
+- **Zero-downtime**: Built-in rolling deployments
+
+### Negative
+
+- **Learning curve**: Need to learn Docker and Kamal basics
+- **Ruby dependency**: Kamal requires Ruby to be installed locally
+- **Resource overhead**: Docker adds ~100-200MB RAM overhead
+- **Build time**: Multi-stage Docker builds take longer than direct deploys
+
+### Neutral
+
+- Obsoletes `docs/vps-deployment.md` (will be deleted)
+- Need to create new documentation for Kamal workflow
+
+## Implementation
+
+1. Create `Dockerfile` with multi-stage build (Node 24)
+2. Create `config/deploy.yml` for Kamal configuration
+3. Set up Traefik for SSL/HTTPS
+4. Use Docker volumes for SQLite persistence
+5. Document in `docs/kamal-deployment.md`
+
+## Future Work
+
+- Add Litestream for SQLite replication (separate epic for HA)
+- Add additional servers for redundancy
+- Set up GitHub Actions for CI/CD with Kamal
+
+## References
+
+- [Kamal Documentation](https://kamal-deploy.org/)
+- [37signals Kamal Announcement](https://world.hey.com/dhh/introducing-kamal-9330a267)
+- [Traefik Documentation](https://doc.traefik.io/traefik/)