perf(tests): copy a migrated template DB instead of migrating per test

Most tests opened a throwaway panel DB with database.InitDB, which runs the
full AutoMigrate + seed on an empty file every time: ~230ms, and ~850ms under
-race because GORM's reflection-heavy migration is what the detector slows
most. internal/web/service does this in ~550 of its 830 tests, so the CI race
job spent ~10 of its ~14.6 minutes re-migrating empty databases.

internal/database/dbtest.InitDB migrates once per test process, then hands
each test its own copy of that file (~130ms under -race) and registers the
CloseDB cleanup. The copy then goes through InitDB like a panel restart, so
every test still starts from the state a fresh install has. Tests that reopen
an existing file, migrate a hand-built legacy DB or target Postgres keep
calling database.InitDB.

Locally under -race: internal/web/service 626s (last CI run) -> 114s,
internal/sub 246s -> 35s.

CLAUDE.md

@@ -154,9 +154,11 @@ file locations when it can answer in one hop.
 - Stdlib `testing` only (no testify). Table-driven, `t.Run` subtests,
   `t.Helper()` on helpers. Assert the exact value / typed error / emitted
   string, never just `err != nil`. Prefer real deps over mocks: throwaway DB via
-  `database.InitDB(filepath.Join(t.TempDir(), "x-ui.db"))` +
-  `t.Cleanup(func() { _ = database.CloseDB() })`; `httptest` for HTTP.
-  `internal/sub`'s `initSubDB(t)` is the template.
+  `dbtest.InitDB(t, filepath.Join(t.TempDir(), "x-ui.db"))`
+  (`internal/database/dbtest`: copies a once-migrated template and registers
+  `CloseDB` cleanup; a fresh `database.InitDB` costs ~7x more, ~850ms under
+  `-race`); `httptest` for HTTP. Keep `database.InitDB` for reopening a file or
+  migrating a hand-built legacy DB. `internal/sub`'s `initSubDB(t)` is the template.
 - Code must pass `golangci-lint run` (gofumpt + goimports formatting): `make lint`.
 - Postgres, xray-gRPC-e2e and scale tests `t.Skip` unless `XUI_TEST_PG_DSN`,
   `XUI_DB_TYPE`+`XUI_DB_DSN`, `XRAY_E2E_BINARY` or `XUI_SCALE_TEST` is set — a