docs: add reference Python/Flask implementation

Previous version of Tanabata used as visual and functional reference
for the new Go + SvelteKit rewrite.

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

docs/reference/backend/internal/storage/postgres/store.go

@@ -0,0 +1,92 @@
+package postgres
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net/http"
+	"time"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgconn"
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+type Storage struct {
+	db *pgxpool.Pool
+}
+
+var connPool *pgxpool.Pool
+
+// Initialize new database storage
+func New(dbURL string) (*Storage, error) {
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	config, err := pgxpool.ParseConfig(dbURL)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse DB URL: %w", err)
+	}
+	config.MaxConns = 10
+	config.MinConns = 2
+	config.HealthCheckPeriod = time.Minute
+	db, err := pgxpool.NewWithConfig(ctx, config)
+	if err != nil {
+		return nil, fmt.Errorf("failed to connect to database: %w", err)
+	}
+	err = db.Ping(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("database ping failed: %w", err)
+	}
+	return &Storage{db: db}, nil
+}
+
+// Close database storage
+func (s *Storage) Close() {
+	s.db.Close()
+}
+
+// Run handler inside transaction
+func (s *Storage) transaction(ctx context.Context, handler func(context.Context, pgx.Tx) (statusCode int, err error)) (statusCode int, err error) {
+	tx, err := connPool.Begin(ctx)
+	if err != nil {
+		statusCode = http.StatusInternalServerError
+		return
+	}
+	statusCode, err = handler(ctx, tx)
+	if err != nil {
+		tx.Rollback(ctx)
+		return
+	}
+	err = tx.Commit(ctx)
+	if err != nil {
+		statusCode = http.StatusInternalServerError
+	}
+	return
+}
+
+// Handle database error
+func (s *Storage) handleDBError(errIn error) (statusCode int, err error) {
+	if errIn == nil {
+		statusCode = http.StatusOK
+		return
+	}
+	if errors.Is(errIn, pgx.ErrNoRows) {
+		err = fmt.Errorf("not found")
+		statusCode = http.StatusNotFound
+		return
+	}
+	var pgErr *pgconn.PgError
+	if errors.As(errIn, &pgErr) {
+		switch pgErr.Code {
+		case "22P02", "22007": // Invalid data format
+			err = fmt.Errorf("%s", pgErr.Message)
+			statusCode = http.StatusBadRequest
+			return
+		case "23505": // Unique constraint violation
+			err = fmt.Errorf("already exists")
+			statusCode = http.StatusConflict
+			return
+		}
+	}
+	return http.StatusInternalServerError, errIn
+}