fix(auth): new users inherit the system-wide default timezone

default_timezone existed as a system setting (admin-editable, exposed
via SettingsRegistry.DefaultTimezone) but nothing read it at user
creation — the CreateUser query didn't even carry the column, so every
new user got the DB default UTC regardless. Both creation paths
(public register + admin create) now seed the user's timezone from the
setting, validated with time.LoadLocation and falling back to UTC when
unwired/empty/invalid.

Test-infra fixes surfaced by the run: TestUpdateProfileSelfEdit was
self-editing the SHARED admin (renaming it past the harness cleanup's
preserve-list, leaking a second admin row that broke the last-admin
tests) — it now edits a dedicated user with a unique name; the
last-admin tests get an ensureSingleAdmin precondition helper; the
websocket collection name is run-unique (non-test names persist in the
shared DB and UNIQUE(user_id,name) 409s on rerun).

internal/database/queries.sql.go

@@ -2003,9 +2003,9 @@ func (q *Queries) CreateUnlinkedBook(ctx context.Context, arg CreateUnlinkedBook
 }
 
 const CreateUser = `-- name: CreateUser :one
-INSERT INTO users (email, username, password_hash, first_name, last_name, theme, role)
-VALUES ($1, $2, $3, $4, $5, $6, $7)
-RETURNING id, email, username, theme, first_name, last_name, role, created_at, updated_at
+INSERT INTO users (email, username, password_hash, first_name, last_name, theme, role, timezone)
+VALUES ($1, $2, $3, $4, $5, $6, $7, $8)
+RETURNING id, email, username, theme, first_name, last_name, role, timezone, created_at, updated_at
 `
 
 type CreateUserParams struct {
@@ -2016,6 +2016,7 @@ type CreateUserParams struct {
 	LastName     pgtype.Text `db:"last_name" json:"last_name"`
 	Theme        pgtype.Text `db:"theme" json:"theme"`
 	Role         string      `db:"role" json:"role"`
+	Timezone     pgtype.Text `db:"timezone" json:"timezone"`
 }
 
 type CreateUserRow struct {
@@ -2026,6 +2027,7 @@ type CreateUserRow struct {
 	FirstName pgtype.Text        `db:"first_name" json:"first_name"`
 	LastName  pgtype.Text        `db:"last_name" json:"last_name"`
 	Role      string             `db:"role" json:"role"`
+	Timezone  pgtype.Text        `db:"timezone" json:"timezone"`
 	CreatedAt pgtype.Timestamptz `db:"created_at" json:"created_at"`
 	UpdatedAt pgtype.Timestamptz `db:"updated_at" json:"updated_at"`
 }
@@ -2039,6 +2041,7 @@ func (q *Queries) CreateUser(ctx context.Context, arg CreateUserParams) (CreateU
 		arg.LastName,
 		arg.Theme,
 		arg.Role,
+		arg.Timezone,
 	)
 	var i CreateUserRow
 	err := row.Scan(
@@ -2049,6 +2052,7 @@ func (q *Queries) CreateUser(ctx context.Context, arg CreateUserParams) (CreateU
 		&i.FirstName,
 		&i.LastName,
 		&i.Role,
+		&i.Timezone,
 		&i.CreatedAt,
 		&i.UpdatedAt,
 	)

internal/database/queries/queries.sql

@@ -1,7 +1,7 @@
 -- name: CreateUser :one
-INSERT INTO users (email, username, password_hash, first_name, last_name, theme, role)
-VALUES ($1, $2, $3, $4, $5, $6, $7)
-RETURNING id, email, username, theme, first_name, last_name, role, created_at, updated_at;
+INSERT INTO users (email, username, password_hash, first_name, last_name, theme, role, timezone)
+VALUES ($1, $2, $3, $4, $5, $6, $7, $8)
+RETURNING id, email, username, theme, first_name, last_name, role, timezone, created_at, updated_at;
 
 -- name: GetUserByEmail :one
 SELECT id, email, username, theme, first_name, last_name, role, created_at, updated_at FROM users WHERE email = $1;

internal/handlers/auth.go

@@ -39,6 +39,23 @@ var SessionDurationSec = int(DefaultSessionDuration.Seconds())
 // SetSettings wires the tunable settings registry (optional).
 func (h *AuthHandler) SetSettings(s *database.SettingsRegistry) { h.settings = s }
 
+// defaultTimezone returns the system-wide default timezone for new users.
+// Falls back to UTC when no registry is wired, the setting is empty, or
+// the value is not a valid IANA location.
+func (h *AuthHandler) defaultTimezone() string {
+	if h.settings == nil {
+		return "UTC"
+	}
+	tz := h.settings.DefaultTimezone()
+	if tz == "" {
+		return "UTC"
+	}
+	if _, err := time.LoadLocation(tz); err != nil {
+		return "UTC"
+	}
+	return tz
+}
+
 // sessionDuration returns the active session duration from the registry.
 func (h *AuthHandler) sessionDuration() time.Duration {
 	if h.settings != nil {
@@ -263,6 +280,7 @@ func (h *AuthHandler) Register(c *echo.Context) error {
 		LastName:     pgtype.Text{String: req.LastName, Valid: req.LastName != ""},
 		Theme:        pgtype.Text{String: "tokyo-night", Valid: true},
 		Role:         userRole,
+		Timezone:     pgtype.Text{String: h.defaultTimezone(), Valid: true},
 	})
 	if err != nil {
 		if c.Request().Header.Get("HX-Request") == "true" {
@@ -735,6 +753,7 @@ func (h *AuthHandler) AdminCreateUser(c *echo.Context) error {
 		LastName:     pgtype.Text{String: req.LastName, Valid: req.LastName != ""},
 		Theme:        pgtype.Text{String: "tokyo-night", Valid: true},
 		Role:         role,
+		Timezone:     pgtype.Text{String: h.defaultTimezone(), Valid: true},
 	})
 	if err != nil {
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
@@ -756,6 +775,7 @@ func (h *AuthHandler) AdminCreateUser(c *echo.Context) error {
 		"email":      user.Email,
 		"username":   user.Username,
 		"role":       user.Role,
+		"timezone":   user.Timezone,
 		"created_at": user.CreatedAt,
 	})
 }