feat: config, migrations embed, and server entrypoint

- internal/config: typed Config struct loaded from env vars via godotenv;
  all fields from docs (listen addr, JWT, DB, storage, thumbs, import)
- migrations/embed.go: embed FS so goose SQL files are baked into the binary
- cmd/server/main.go: load config → connect pgxpool → goose migrations
  (embedded) → Gin server with GET /health returning 200 OK
- .env.example: documents all required and optional env vars
- go.mod: bump to Go 1.26, add gin/pgx/goose/godotenv as direct deps

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

backend/cmd/server/main.go

@@ -0,0 +1,66 @@
+package main
+
+import (
+	"context"
+	"log/slog"
+	"net/http"
+	"os"
+
+	"github.com/gin-gonic/gin"
+	"github.com/jackc/pgx/v5/pgxpool"
+	"github.com/jackc/pgx/v5/stdlib"
+	"github.com/pressly/goose/v3"
+
+	"tanabata/backend/internal/config"
+	"tanabata/backend/migrations"
+)
+
+func main() {
+	cfg, err := config.Load()
+	if err != nil {
+		slog.Error("failed to load config", "err", err)
+		os.Exit(1)
+	}
+
+	pool, err := pgxpool.New(context.Background(), cfg.DatabaseURL)
+	if err != nil {
+		slog.Error("failed to connect to database", "err", err)
+		os.Exit(1)
+	}
+	defer pool.Close()
+
+	if err := pool.Ping(context.Background()); err != nil {
+		slog.Error("database ping failed", "err", err)
+		os.Exit(1)
+	}
+	slog.Info("database connected")
+
+	// Run migrations using the embedded FS.
+	// stdlib.OpenDBFromPool wraps the pool in a *sql.DB without closing
+	// the pool when the sql.DB is closed.
+	migDB := stdlib.OpenDBFromPool(pool)
+	goose.SetBaseFS(migrations.FS)
+	if err := goose.SetDialect("postgres"); err != nil {
+		slog.Error("goose dialect error", "err", err)
+		os.Exit(1)
+	}
+	if err := goose.Up(migDB, "."); err != nil {
+		slog.Error("migrations failed", "err", err)
+		os.Exit(1)
+	}
+	migDB.Close()
+	slog.Info("migrations applied")
+
+	r := gin.New()
+	r.Use(gin.Recovery())
+
+	r.GET("/health", func(c *gin.Context) {
+		c.Status(http.StatusOK)
+	})
+
+	slog.Info("starting server", "addr", cfg.ListenAddr)
+	if err := r.Run(cfg.ListenAddr); err != nil {
+		slog.Error("server error", "err", err)
+		os.Exit(1)
+	}
+}