test: add dbtest.ResetDB helper that preserves admin user

Integration tests previously TRUNCATEd the users table at setup,
which wiped the operator's admin login every time the suite ran
against a Postgres instance shared with their dev environment.

This commit:

- Adds internal/dbtest/reset.go exposing ResetDB(t, pool) and
  TestUserPrefix. ResetDB truncates every data table EXCEPT users,
  then deletes only users whose username starts with TestUserPrefix.
- Migrates 9 test files (subsonic/scrobble, subsonic/star,
  recommendation/candidates, scrobble/queue, similarity/worker,
  playevents/writer, playsessions/service, api/auth, api/me) to
  call ResetDB instead of issuing TRUNCATE-with-users.
- Renames hardcoded test usernames to use TestUserPrefix so ResetDB
  cleans them between runs. seedUser in api/auth_test now prefixes
  internally; existing call sites pass bare names.
- Leaves auth/bootstrap_test.go alone — it legitimately tests
  first-time admin bootstrap and must wipe users.

Verified locally: full integration suite with -p 1 runs cleanly
under the existing MINSTREL_TEST_DATABASE_URL setup, and the admin
row in the shared dev DB survives the run.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/api/me_test.go

@@ -26,8 +26,8 @@ func TestHandleGetMe_ReturnsAuthenticatedUser(t *testing.T) {
 	if err := json.Unmarshal(w.Body.Bytes(), &got); err != nil {
 		t.Fatalf("decode: %v", err)
 	}
-	if got.Username != "alice" || !got.IsAdmin {
-		t.Errorf("user = %+v, want alice/admin", got)
+	if got.Username != "test-alice" || !got.IsAdmin {
+		t.Errorf("user = %+v, want test-alice/admin", got)
 	}
 }