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/playlists_system_test.go

@@ -8,8 +8,6 @@ import (
 	"testing"
 
 	"github.com/go-chi/chi/v5"
-
-	"git.fabledsword.com/bvandeusen/minstrel/internal/auth"
 )
 
 // Generic registry-driven system-playlist endpoints (#411 R2).
@@ -17,7 +15,11 @@ import (
 func newSystemPlaylistRouter(h *handlers) chi.Router {
 	r := chi.NewRouter()
 	r.Route("/api", func(api chi.Router) {
-		api.Use(auth.RequireUser(h.pool))
+		// No real auth.RequireUser middleware: like every other api
+		// handler test, auth is supplied via withUser() context and the
+		// handlers self-guard with requireUser() (prelude.go). The real
+		// middleware needs a live session and would 401 the injected
+		// test user.
 		api.Post("/playlists/system/{kind}/refresh", h.handleSystemPlaylistRefresh)
 		api.Get("/playlists/system/{kind}/shuffle", h.handleSystemPlaylistShuffle)
 	})