fix: A2 (relax art-source CHECK) + D + E integration residual

A2 — migration 0030: the albums/artists art `*_source` CHECK was a
fixed provider-ID allowlist fighting the extensible coverart.Register
registry (per-provider migration churn at 0016/0018/0020; rejected
test stub providers → ~11 enricher tests failed). Relax to "NULL or
non-empty"; the registry is the source of truth. Same brittleness
class as the #433 discovery-mix CHECK.

D — lidarr Approve resolves metadata/quality profiles (JSON arrays)
before the add; the test stubs returned {"id":1} for every path, so
ListMetadataProfiles failed to unmarshal → 500/Approve errors. Make
the lidarrrequests + admin_requests stubs path-aware (arrays for
/metadataprofile, /qualityprofile).

E:
- auth: requireUser (prelude.go) emitted code "auth_required"; the
  canonical code is "unauthenticated" (operator decision). Change the
  code; drop the now-dead "auth_required" web error-copy key
  ("unauthenticated" already has copy).
- playlists_system_test wrapped the real auth.RequireUser middleware
  but only injected withUser() context → 401. Like every other api
  handler test, drop the middleware and rely on requireUser().
- admin_users dup-username test seeded "test-existing" (seedUser
  prefixes) but POSTed "existing" → no collision; POST the prefixed
  name.
- me_timezone test decoded a top-level {"code"} but the envelope is
  {"error":{"code"}}; decode the nested shape.
- audit test asserted compact JSON; Postgres jsonb::text is spaced.
- put-lidarr-config tests predated the missing_defaults gate (correct
  handler behavior); supply the required defaults in the bodies.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/api/admin_users_test.go

@@ -163,7 +163,10 @@ func TestAdminCreateUser_DuplicateUsername_409(t *testing.T) {
 	admin := seedUser(t, pool, "duper", "pw", true)
 	seedUser(t, pool, "existing", "pw", false)
 
-	body := `{"username":"existing","password":"abcd1234"}`
+	// seedUser prefixes usernames with dbtest.TestUserPrefix, so the
+	// row above is "test-existing"; POST that exact name to actually
+	// collide (stays test-prefixed for ResetDB).
+	body := `{"username":"test-existing","password":"abcd1234"}`
 	req := httptest.NewRequest(http.MethodPost, "/api/admin/users", bytes.NewReader([]byte(body)))
 	req = withUser(req, admin)
 	rec := httptest.NewRecorder()