refactor: restructure internal/ into adapters/, infra/, and app layers

- internal/gemini/ → internal/adapters/openai/ (renamed package to openai)
- internal/pexels/ → internal/adapters/pexels/
- internal/config/   → internal/infra/config/
- internal/database/ → internal/infra/database/
- internal/locale/   → internal/infra/locale/
- internal/middleware/ → internal/infra/middleware/
- internal/server/   → internal/infra/server/

All import paths and call sites updated accordingly.

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

backend/internal/infra/config/config.go

@@ -0,0 +1,35 @@
+package config
+
+import (
+	"time"
+
+	"github.com/kelseyhightower/envconfig"
+)
+
+type Config struct {
+	Port        int    `envconfig:"PORT" default:"8080"`
+	DatabaseURL string `envconfig:"DATABASE_URL" required:"true"`
+
+	// Firebase
+	FirebaseCredentialsFile string `envconfig:"FIREBASE_CREDENTIALS_FILE" required:"true"`
+
+	// JWT
+	JWTSecret          string        `envconfig:"JWT_SECRET" required:"true"`
+	JWTAccessDuration  time.Duration `envconfig:"JWT_ACCESS_DURATION" default:"15m"`
+	JWTRefreshDuration time.Duration `envconfig:"JWT_REFRESH_DURATION" default:"720h"`
+
+	// CORS
+	AllowedOrigins []string `envconfig:"ALLOWED_ORIGINS" default:"http://localhost:3000"`
+
+	// External APIs
+	OpenAIAPIKey  string `envconfig:"OPENAI_API_KEY" required:"true"`
+	PexelsAPIKey  string `envconfig:"PEXELS_API_KEY" required:"true"`
+}
+
+func Load() (*Config, error) {
+	var cfg Config
+	if err := envconfig.Process("", &cfg); err != nil {
+		return nil, err
+	}
+	return &cfg, nil
+}