refactor(store): single account name on users; rename profile/laps tables

users.display_name becomes users.name and is now the only human-facing
name -- profiles.name is dropped (it duplicated the account name; the
Profile page's Name field now edits users.name through PUT /api/profile,
which carries Name alongside the profiles columns). The profile table
becomes profiles and laps becomes activity_laps, homogeneous with
activity_samples. Onboarding pre-fills the display name from the OIDC
claim's name.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/store/users.go

@@ -11,18 +11,18 @@ import (
 // state) is scoped to exactly one User -- see
 // docs/superpowers/specs/2026-07-25-per-user-profile-design.md.
 type User struct {
-	ID          int64
-	OIDCSub     string
-	DisplayName string
-	CreatedAt   string
+	ID        int64
+	OIDCSub   string
+	Name      string
+	CreatedAt string
 }
 
 // GetUserBySub looks up a user by their OIDC subject -- the only lookup key
 // the session-resolution middleware ever uses.
 func (db *DB) GetUserBySub(ctx context.Context, oidcSub string) (User, bool, error) {
 	var u User
-	err := db.QueryRowContext(ctx, `SELECT id, oidc_sub, display_name, created_at FROM users WHERE oidc_sub = ?`, oidcSub).
-		Scan(&u.ID, &u.OIDCSub, &u.DisplayName, &u.CreatedAt)
+	err := db.QueryRowContext(ctx, `SELECT id, oidc_sub, name, created_at FROM users WHERE oidc_sub = ?`, oidcSub).
+		Scan(&u.ID, &u.OIDCSub, &u.Name, &u.CreatedAt)
 	if err == sql.ErrNoRows {
 		return User{}, false, nil
 	}
@@ -63,7 +63,7 @@ func (db *DB) ProvisionUser(ctx context.Context, oidcSub, displayName string) (i
 	}
 	defer tx.Rollback()
 
-	res, err := tx.ExecContext(ctx, `INSERT INTO users (oidc_sub, display_name) VALUES (?, ?)`, oidcSub, displayName)
+	res, err := tx.ExecContext(ctx, `INSERT INTO users (oidc_sub, name) VALUES (?, ?)`, oidcSub, displayName)
 	if err != nil {
 		return 0, fmt.Errorf("create user %q: %w", oidcSub, err)
 	}
@@ -72,7 +72,7 @@ func (db *DB) ProvisionUser(ctx context.Context, oidcSub, displayName string) (i
 		return 0, err
 	}
 
-	if _, err := tx.ExecContext(ctx, `INSERT INTO profile (user_id, name) VALUES (?, ?)`, userID, displayName); err != nil {
+	if _, err := tx.ExecContext(ctx, `INSERT INTO profiles (user_id) VALUES (?)`, userID); err != nil {
 		return 0, fmt.Errorf("create profile for user %d: %w", userID, err)
 	}
 
@@ -113,3 +113,12 @@ func (db *DB) DeleteUser(ctx context.Context, userID int64) error {
 	}
 	return nil
 }
+
+// UpdateUserName renames userID's account -- the single human-facing name
+// (shown in the header and session info), editable from the Profile page.
+func (db *DB) UpdateUserName(ctx context.Context, userID int64, name string) error {
+	if _, err := db.ExecContext(ctx, `UPDATE users SET name = ? WHERE id = ?`, name, userID); err != nil {
+		return fmt.Errorf("update name for user %d: %w", userID, err)
+	}
+	return nil
+}