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/cmd/migrate/main.go

@@ -0,0 +1,105 @@
+// Package main is the migration runner. It applies embedded SQL migrations to the
+// configured database. Intended to run as a one-shot container/job before the API.
+//
+// Usage: migrate [up|down|version|force <n>]
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log/slog"
+	"os"
+	"time"
+
+	"github.com/golang-migrate/migrate/v4"
+	_ "github.com/golang-migrate/migrate/v4/database/postgres"
+	"github.com/golang-migrate/migrate/v4/source/iofs"
+
+	"github.com/mitbringsl/backend/internal/config"
+	"github.com/mitbringsl/backend/internal/logging"
+	"github.com/mitbringsl/backend/migrations"
+)
+
+func main() {
+	cfg, err := config.Load()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "config: %v\n", err)
+		os.Exit(1)
+	}
+	logging.Init(cfg.LogLevel)
+
+	cmd := "up"
+	if len(os.Args) > 1 {
+		cmd = os.Args[1]
+	}
+
+	if err := run(cfg, cmd); err != nil {
+		slog.Error("migration failed", "cmd", cmd, "error", err)
+		os.Exit(1)
+	}
+}
+
+func run(cfg *config.Config, cmd string) error {
+	src, err := iofs.New(migrations.FS, ".")
+	if err != nil {
+		return fmt.Errorf("create source: %w", err)
+	}
+
+	// Use the configured DATABASE_URL; golang-migrate accepts the same DSN
+	// form pgx uses (postgres://...). Allow a short startup grace period.
+	dbURL := cfg.DatabaseURL
+
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	var m *migrate.Migrate
+	var lastErr error
+	for attempt := 0; attempt < 30; attempt++ {
+		m, lastErr = migrate.NewWithSourceInstance("iofs", src, dbURL)
+		if lastErr == nil {
+			break
+		}
+		select {
+		case <-ctx.Done():
+			return fmt.Errorf("db not ready: %w", lastErr)
+		case <-time.After(time.Second):
+		}
+	}
+	if lastErr != nil {
+		return fmt.Errorf("create migrate instance: %w", lastErr)
+	}
+	defer m.Close()
+
+	switch cmd {
+	case "up":
+		if err := m.Up(); err != nil && !errors.Is(err, migrate.ErrNoChange) {
+			return err
+		}
+		slog.Info("migrations applied (up)")
+	case "down":
+		if err := m.Down(); err != nil && !errors.Is(err, migrate.ErrNoChange) {
+			return err
+		}
+		slog.Info("migrations applied (down)")
+	case "version":
+		v, dirty, err := m.Version()
+		if err != nil {
+			return err
+		}
+		fmt.Printf("version=%d dirty=%v\n", v, dirty)
+	case "force":
+		if len(os.Args) < 3 {
+			return errors.New("force requires a version argument")
+		}
+		var v int
+		fmt.Sscanf(os.Args[2], "%d", &v)
+		if err := m.Force(v); err != nil {
+			return err
+		}
+		slog.Info("forced migration version", "version", v)
+	default:
+		return fmt.Errorf("unknown command %q (use up|down|version|force)", cmd)
+	}
+	return nil
+}