fix(database): create the first panel user with random credentials

The first user is now created when the server starts, with a random
username and password printed once to its log (docker logs, the journal),
instead of a fixed pair seeded by InitDB. CLI calls that initialize the
database are often piped away, as the Docker entrypoint's port lookup is, so
only the server's own log reliably reaches the operator.

`x-ui setting -show` keeps reporting hasDefaultCredential: true until a user
exists, so install.sh still sets its own credentials on a fresh install.
Existing databases are not changed. Verified on a built binary: fresh
setting -show, install.sh-style credential set, first start, login and
restart. The PostgreSQL lane was not run locally; the change adds no query.

.oss-scanner/threat_model.md

@@ -69,8 +69,8 @@ responses for limited tokens, or node sync payloads.
 - Frontend: `cd /src/frontend && npx vitest run --project unit --project components`.
   The Storybook browser project needs Chromium and is not provisioned.
 - Running a live panel: `XUI_DB_FOLDER=/tmp/xui XUI_LOG_FOLDER=/tmp/xui
-  XUI_BIN_FOLDER=/src/build/bin x-ui` (creates `admin`/`admin` on first start;
-  `x-ui setting -h` changes port, base path and credentials).
+  XUI_BIN_FOLDER=/src/build/bin x-ui` (logs a generated username and password on
+  first start; `x-ui setting -h` changes port, base path and credentials).
 - Baseline logs from the image build: `/src/.oss-scanner-go-test.log` and
   `/src/.oss-scanner-vitest.log`. A built image does not prove the tests passed.
 - PostgreSQL is not installed: tests gated on `XUI_TEST_PG_DSN` skip, so a