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/go.mod

@@ -0,0 +1,21 @@
+module github.com/mitbringsl/backend
+
+go 1.26
+
+require (
+	github.com/caarlos0/env/v11 v11.4.1
+	github.com/golang-migrate/migrate/v4 v4.18.2
+	github.com/jackc/pgx/v5 v5.10.0
+)
+
+require (
+	github.com/hashicorp/errwrap v1.1.0 // indirect
+	github.com/hashicorp/go-multierror v1.1.1 // indirect
+	github.com/jackc/pgpassfile v1.0.0 // indirect
+	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
+	github.com/jackc/puddle/v2 v2.2.2 // indirect
+	github.com/lib/pq v1.10.9 // indirect
+	go.uber.org/atomic v1.7.0 // indirect
+	golang.org/x/sync v0.17.0 // indirect
+	golang.org/x/text v0.29.0 // indirect
+)