Add public feed, admin area, self-registration, visibility & hashtags

- Public feed (/) with infinite scroll via Intersection Observer
- Self-registration (/register)
- Admin area (/admin/entries, /admin/users) with user management
- journal_entries: visibility (public/private) + hashtags fields
- users: is_admin flag
- DB schema updated (recreate DB to apply)
- CI: run go test via docker run (golang:1.25-alpine) — fixes 'go not found'

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

backend/internal/db/users.go

@@ -0,0 +1,43 @@
+package db
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"github.com/jacek/pamietnik/backend/internal/domain"
+)
+
+type UserStore struct {
+	pool *pgxpool.Pool
+}
+
+func NewUserStore(pool *pgxpool.Pool) *UserStore {
+	return &UserStore{pool: pool}
+}
+
+// ListUsers returns all users ordered by created_at.
+func (s *UserStore) ListUsers(ctx context.Context) ([]domain.User, error) {
+	rows, err := s.pool.Query(ctx,
+		`SELECT user_id, username, is_admin, created_at FROM users ORDER BY created_at`,
+	)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var users []domain.User
+	for rows.Next() {
+		var u domain.User
+		if err := rows.Scan(&u.UserID, &u.Username, &u.IsAdmin, &u.CreatedAt); err != nil {
+			return nil, err
+		}
+		users = append(users, u)
+	}
+	return users, rows.Err()
+}
+
+// DeleteUser removes a user by ID.
+func (s *UserStore) DeleteUser(ctx context.Context, userID string) error {
+	_, err := s.pool.Exec(ctx, `DELETE FROM users WHERE user_id = $1`, userID)
+	return err
+}