Backend Phase A: foundation, migrations, Docker setup

- Go backend skeleton: config (caarlos0/env), slog JSON logging,
  pgxpool store, HTTP server with graceful shutdown.
- httpapi: render helpers, Problem errors, middleware chain
  (requestID / logging / recover / CORS), /healthz and /readyz.
- Migrations: full initial schema (users, sessions, lists,
  list_members, items, op_log SOURCE OF TRUTH, item_names) +
  golang-migrate runner binary using source/iofs (embedded).
- Docker: multi-stage Dockerfile (Go 1.26 -> distroless nonroot),
  builds both server and migrate binaries.
- deploy: docker-compose (caddy + backend + migrate + postgres:16),
  Caddyfile (auto-HTTPS), .env.example, pg extensions init script.
- AGENTS.md: project context + roadmap for AI agents.

Verified: image builds, both binaries run in container (smoke test).

backend/migrations/000001_init_schema.down.sql

@@ -0,0 +1,8 @@
+-- Reverse of 000001.
+DROP TABLE IF EXISTS item_names;
+DROP TABLE IF EXISTS op_log;
+DROP TABLE IF EXISTS items;
+DROP TABLE IF EXISTS list_members;
+DROP TABLE IF EXISTS lists;
+DROP TABLE IF EXISTS sessions;
+DROP TABLE IF EXISTS users;