Backend Phase B (1/2): password auth + sessions

Argon2id password hashing (PHC format, self-encoded/decoded without an
external lib) with constant-time verification, UserStore (create/get by
email and id) and SessionStore (opaque crypto/rand tokens, SHA-256 hashed
in DB, create/lookup/revoke, last_seen_at bump on lookup).

HTTP layer: Register/Login/Logout handlers + RequireAuth middleware.
Login uses a dummy-hash path so unknown-email and wrong-password yield the
same timing/shape, narrowing user enumeration. Tokens accepted via Bearer
header (native clients) or session cookie (HttpOnly, SameSite=Lax).

Routes wired in api.go: POST /auth/register, /auth/login, /auth/logout.
Verified with go test, go vet and an end-to-end smoke test against a real
PostgreSQL container (register/login/logout/duplicate/short-pw/wrong-pw).

OIDC (Phase B part 2) follows next; the issueSession helper is reused.

backend/internal/auth/user.go

@@ -0,0 +1,95 @@
+package auth
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+// User is the application-side view of a row in the users table.
+type User struct {
+	ID           uuid.UUID
+	Email        string
+	PasswordHash *string // nil for OIDC-only accounts
+	OIDCSubject  *string
+	OIDCIssuer   *string
+	DisplayName  *string
+}
+
+// ErrUserNotFound is returned when no user matches the query.
+var ErrUserNotFound = errors.New("user not found")
+
+// UserStore wraps database access for the users table.
+type UserStore struct {
+	pool *pgxpool.Pool
+}
+
+// NewUserStore constructs a UserStore backed by the given pool.
+func NewUserStore(pool *pgxpool.Pool) *UserStore {
+	return &UserStore{pool: pool}
+}
+
+// CreateUser inserts a new user with a password hash (email/password accounts).
+// It returns the created user. Email uniqueness violations surface as ErrEmailTaken.
+var ErrEmailTaken = errors.New("email already registered")
+
+func (s *UserStore) CreateUser(ctx context.Context, email, passwordHash, displayName string) (User, error) {
+	const q = `
+		INSERT INTO users (email, password_hash, display_name)
+		VALUES ($1, $2, NULLIF($3, ''))
+		RETURNING id, email, password_hash, oidc_subject, oidc_issuer, display_name`
+	var u User
+	var dn *string
+	err := s.pool.QueryRow(ctx, q, email, passwordHash, displayName).
+		Scan(&u.ID, &u.Email, &u.PasswordHash, &u.OIDCSubject, &u.OIDCIssuer, &dn)
+	if err != nil {
+		if isUniqueViolation(err) {
+			return User{}, fmt.Errorf("%w: %s", ErrEmailTaken, email)
+		}
+		return User{}, fmt.Errorf("create user: %w", err)
+	}
+	u.DisplayName = dn
+	return u, nil
+}
+
+// GetUserByEmail loads a user by its (case-sensitive) email address.
+func (s *UserStore) GetUserByEmail(ctx context.Context, email string) (User, error) {
+	const q = `
+		SELECT id, email, password_hash, oidc_subject, oidc_issuer, display_name
+		FROM users WHERE email = $1`
+	var u User
+	var dn *string
+	err := s.pool.QueryRow(ctx, q, email).
+		Scan(&u.ID, &u.Email, &u.PasswordHash, &u.OIDCSubject, &u.OIDCIssuer, &dn)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return User{}, ErrUserNotFound
+		}
+		return User{}, fmt.Errorf("get user by email: %w", err)
+	}
+	u.DisplayName = dn
+	return u, nil
+}
+
+// GetUserByID loads a user by its primary key.
+func (s *UserStore) GetUserByID(ctx context.Context, id uuid.UUID) (User, error) {
+	const q = `
+		SELECT id, email, password_hash, oidc_subject, oidc_issuer, display_name
+		FROM users WHERE id = $1`
+	var u User
+	var dn *string
+	err := s.pool.QueryRow(ctx, q, id).
+		Scan(&u.ID, &u.Email, &u.PasswordHash, &u.OIDCSubject, &u.OIDCIssuer, &dn)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return User{}, ErrUserNotFound
+		}
+		return User{}, fmt.Errorf("get user by id: %w", err)
+	}
+	u.DisplayName = dn
+	return u, nil
+}