infra: add Docker setup for portable deployment

Dockerfile (Python 3.12 slim) + docker-compose (backend + nginx).
Backend on port 8000 inside container, nginx proxies API and serves
frontend static. SQLite persisted in named volume. Nginx listens on
127.0.0.1:8080 — external SSL handled by host reverse proxy.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,16 @@
+.git
+.gitignore
+.env
+.venv
+venv
+__pycache__
+*.pyc
+*.db
+tests/
+docs/
+deploy/
+frontend/
+nginx/
+*.md
+.kin_worktrees/
+PROGRESS.md