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/database/postgres.go

@@ -0,0 +1,32 @@
+package database
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+func NewPool(ctx context.Context, databaseURL string) (*pgxpool.Pool, error) {
+	config, err := pgxpool.ParseConfig(databaseURL)
+	if err != nil {
+		return nil, fmt.Errorf("parse config: %w", err)
+	}
+
+	config.MaxConns = 20
+	config.MinConns = 5
+	config.MaxConnLifetime = 30 * time.Minute
+	config.MaxConnIdleTime = 5 * time.Minute
+
+	pool, err := pgxpool.NewWithConfig(ctx, config)
+	if err != nil {
+		return nil, fmt.Errorf("create pool: %w", err)
+	}
+
+	if err := pool.Ping(ctx); err != nil {
+		return nil, fmt.Errorf("ping: %w", err)
+	}
+
+	return pool, nil
+}