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/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;