feat: multi-architecture Docker setup (x86_64 + ARM64) with one-command install

- Multi-stage Dockerfile: builder compiles native modules (better-sqlite3,
  tiny-secp256k1) under target architecture, runtime is minimal Alpine
- install.sh: POSIX sh installer (Alpine ash compatible) with architecture
  detection, Docker install, .env validation, health-check retry loop
- docker-compose.yml: removed platform locks, .env read-only mount,
  127.0.0.1 port binding, 384m mem limit (Orange Pi Zero 2 safe)
- .dockerignore: excludes node_modules, secrets, tests, .kilo
- README.md: complete rewrite with deployment docs for any device
- Verified: POSIX sh syntax (dash), Dockerfile (docker build --check),
  docker-compose (docker compose config)

.dockerignore

@@ -0,0 +1,18 @@
+node_modules
+db
+uploads
+.env
+.env.*
+!.env.example
+.git
+.kilo
+.architect
+kilo-meta.json
+kilo.jsonc
+AGENTS.md
+corrupt-photo.jpg
+wg/config
+*.log
+__pycache__
+**/__tests__
+**/*.test.js