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/dbtest/reset.go

@@ -0,0 +1,73 @@
+// Package dbtest provides shared helpers for integration tests that need
+// a clean Postgres state without disturbing the operator's admin user.
+//
+// Background: when integration tests run against the same Postgres
+// instance an operator uses for local dev (a common dev-laptop setup),
+// a TRUNCATE that includes the users table wipes the operator's admin
+// login between every test run. To avoid that, tests should:
+//
+//  1. Call ResetDB instead of issuing TRUNCATE statements directly. It
+//     truncates every data table EXCEPT users, then deletes only those
+//     users whose username starts with TestUserPrefix.
+//  2. Always create test user rows with a username that begins with
+//     TestUserPrefix; otherwise those rows survive across runs and
+//     leak into other tests.
+//
+// One test legitimately needs to wipe the entire users table:
+// internal/auth/bootstrap_test.go, which exercises first-time admin
+// bootstrap. That file does its own TRUNCATE and does not use this
+// helper.
+package dbtest
+
+import (
+	"context"
+	"strings"
+	"testing"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+// TestUserPrefix is the required username prefix for any user row
+// created by an integration test. ResetDB removes every user matching
+// this prefix and leaves all others intact.
+const TestUserPrefix = "test-"
+
+// dataTables is the union of every non-users table touched by any
+// integration test in the repo. Truncating the union is harmless for
+// callers that only care about a subset; truncating tables that don't
+// exist would error, but every name here corresponds to a migration
+// that has shipped.
+var dataTables = []string{
+	"artist_similarity",
+	"track_similarity",
+	"scrobble_queue",
+	"contextual_likes",
+	"general_likes_albums",
+	"general_likes_artists",
+	"general_likes",
+	"play_events",
+	"skip_events",
+	"play_sessions",
+	"sessions",
+	"tracks",
+	"albums",
+	"artists",
+}
+
+// ResetDB clears all data tables and removes any user whose username
+// begins with TestUserPrefix. Users without that prefix (notably the
+// operator's admin row) are left alone. Calls t.Fatalf on error.
+func ResetDB(t *testing.T, pool *pgxpool.Pool) {
+	t.Helper()
+	ctx := context.Background()
+	stmt := "TRUNCATE " + strings.Join(dataTables, ", ") + " RESTART IDENTITY CASCADE"
+	if _, err := pool.Exec(ctx, stmt); err != nil {
+		t.Fatalf("dbtest.ResetDB truncate: %v", err)
+	}
+	if _, err := pool.Exec(ctx,
+		"DELETE FROM users WHERE username LIKE $1",
+		TestUserPrefix+"%",
+	); err != nil {
+		t.Fatalf("dbtest.ResetDB delete test users: %v", err)
+	}
+}