feat: initial commit — backend API + student cabinet frontend

- Go backend: auth (JWT), points earn/spend, QR token generation,
  partners, admin grant/stats endpoints with chi router
- Next.js 14 frontend: login, student dashboard, transaction history,
  QR display, partners list
- PostgreSQL migrations (4 tables), Redis cache, Docker Compose
- CORS middleware, role-based route protection, Zustand auth store

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

backend/internal/users/repository.go

@@ -0,0 +1,107 @@
+package users
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+// ErrNotFound is returned when the requested user does not exist.
+var ErrNotFound = errors.New("not found")
+
+// Repository handles all database access for the users domain.
+type Repository struct {
+	db *pgxpool.Pool
+}
+
+// NewRepository creates a new users Repository.
+func NewRepository(db *pgxpool.Pool) *Repository {
+	return &Repository{db: db}
+}
+
+// GetByID fetches a user's profile by primary key.
+// Returns ErrNotFound if no user exists with that ID.
+func (r *Repository) GetByID(ctx context.Context, id string) (*Profile, error) {
+	var p Profile
+	err := r.db.QueryRow(ctx,
+		`SELECT id, email, name, COALESCE(student_id, ''), balance
+		 FROM users WHERE id = $1`,
+		id,
+	).Scan(&p.ID, &p.Email, &p.Name, &p.StudentID, &p.Balance)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return nil, ErrNotFound
+		}
+		return nil, fmt.Errorf("repository.GetByID: %w", err)
+	}
+	return &p, nil
+}
+
+// UpdateBalance adds delta to the user's balance within an existing pgx transaction.
+// delta is positive when earning points, negative when spending.
+// Returns the new balance after the update via RETURNING, so the service can include
+// it in the API response without a second query.
+// The database CHECK (balance >= 0) acts as the last line of defense against overdrafts;
+// this function will return an error if the constraint fires.
+func (r *Repository) UpdateBalance(ctx context.Context, tx pgx.Tx, id string, delta int) (int, error) {
+	var newBalance int
+	err := tx.QueryRow(ctx,
+		`UPDATE users SET balance = balance + $1 WHERE id = $2 RETURNING balance`,
+		delta, id,
+	).Scan(&newBalance)
+	if err != nil {
+		return 0, fmt.Errorf("repository.UpdateBalance: %w", err)
+	}
+	return newBalance, nil
+}
+
+// CountTransactions returns the total number of transactions for the given user.
+// Used alongside ListTransactions to populate pagination metadata.
+func (r *Repository) CountTransactions(ctx context.Context, userID string) (int, error) {
+	var count int
+	err := r.db.QueryRow(ctx,
+		`SELECT COUNT(*) FROM transactions WHERE user_id = $1`,
+		userID,
+	).Scan(&count)
+	if err != nil {
+		return 0, fmt.Errorf("repository.CountTransactions: %w", err)
+	}
+	return count, nil
+}
+
+// ListTransactions returns paginated transactions for the given user, ordered newest first.
+func (r *Repository) ListTransactions(ctx context.Context, userID string, limit, offset int) ([]Transaction, error) {
+	rows, err := r.db.Query(ctx, `
+		SELECT id,
+		       amount,
+		       type,
+		       COALESCE(description, ''),
+		       COALESCE(partner_id::text, ''),
+		       created_at
+		FROM transactions
+		WHERE user_id = $1
+		ORDER BY created_at DESC
+		LIMIT $2 OFFSET $3`,
+		userID, limit, offset,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("repository.ListTransactions: %w", err)
+	}
+	defer rows.Close()
+
+	var txs []Transaction
+	for rows.Next() {
+		var t Transaction
+		if err := rows.Scan(&t.ID, &t.Amount, &t.Type, &t.Description, &t.PartnerID, &t.CreatedAt); err != nil {
+			return nil, fmt.Errorf("repository.ListTransactions: scan: %w", err)
+		}
+		txs = append(txs, t)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, fmt.Errorf("repository.ListTransactions: rows: %w", err)
+	}
+	return txs, nil
+}