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/playsessions/service_test.go

@@ -13,6 +13,7 @@ import (
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/dbtest"
 )
 
 func testPool(t *testing.T) *pgxpool.Pool {
@@ -32,17 +33,14 @@ func testPool(t *testing.T) *pgxpool.Pool {
 		t.Fatalf("pool: %v", err)
 	}
 	t.Cleanup(pool.Close)
-	if _, err := pool.Exec(context.Background(),
-		"TRUNCATE play_events, skip_events, play_sessions, sessions, users RESTART IDENTITY CASCADE"); err != nil {
-		t.Fatalf("truncate: %v", err)
-	}
+	dbtest.ResetDB(t, pool)
 	return pool
 }
 
 func seedTestUser(t *testing.T, pool *pgxpool.Pool) pgtype.UUID {
 	t.Helper()
 	u, err := dbq.New(pool).CreateUser(context.Background(), dbq.CreateUserParams{
-		Username:     "tester",
+		Username:     dbtest.TestUserPrefix + "tester",
 		PasswordHash: "x",
 		ApiToken:     "x",
 		IsAdmin:      false,