feat: operator-defined model aliases

Spec §5 (v0.2): Config.ModelAliases map[string]string, alias-first
resolution precedence ahead of provider-prefix splitting, alias-OR-target
authorization, lazy target resolution with a 404 naming both ids,
/v1/models listing, admin overview alias table (JSON + dashboard), and
construction-time validation (shape, prefix-shadow, explicit-model
collision, chain, empty target).

routes_unified.go/routes_media.go needed no resolution-call-site changes
- both already pass the client's raw requested id through to
registry.resolve, so aliases inherit the __alias echo and multipart
rewrite machinery for free. cache.go needed no changes either - cacheKey
already hashes requestedModel, so two aliases resolving to the same
target already produce distinct cache entries.

Adds registry_test.go validation table + precedence/authz/lazy-404/
listing coverage, a dedicated cache-key split test, chat/embeddings/
images/audio-transcriptions end-to-end tests, an extended admin overview
test, and an integration test covering the openai-type vs anthropic-type
echo asymmetry under real Traefik+Yaegi.

admin_test.go

@@ -195,6 +195,14 @@ func assertOAIErrorEnvelope(t *testing.T, rec *httptest.ResponseRecorder) {
 func TestAdminOverview_SortedShapeAndVersion(t *testing.T) {
 	t.Parallel()
 	cfg := newAdminTestConfig()
+	// Two aliases, deliberately out of sorted order, proving both the
+	// overview's aliases table (spec §5, v0.2) is populated AND sorted by
+	// alias — "zaliased/x" is a bare target ("z-model", zeta's own
+	// explicit model), "aaliased/y" is provider-prefixed.
+	cfg.ModelAliases = map[string]string{
+		"zaliased/x": "z-model",
+		"aaliased/y": "alpha/a-model-1",
+	}
 	h, _ := newAdminGatewayHandle(t, cfg)
 
 	rec := httptest.NewRecorder()
@@ -247,6 +255,19 @@ func TestAdminOverview_SortedShapeAndVersion(t *testing.T) {
 	if got.Redis.Configured {
 		t.Error("redis.configured must be false: cfg.Redis was never configured")
 	}
+
+	wantAliases := []adminAliasView{
+		{Alias: "aaliased/y", Target: "alpha/a-model-1"},
+		{Alias: "zaliased/x", Target: "z-model"},
+	}
+	if len(got.Aliases) != len(wantAliases) {
+		t.Fatalf("aliases = %+v, want exactly %+v", got.Aliases, wantAliases)
+	}
+	for i, want := range wantAliases {
+		if got.Aliases[i] != want {
+			t.Errorf("aliases[%d] = %+v, want %+v (sorted by alias)", i, got.Aliases[i], want)
+		}
+	}
 }
 
 func TestAdminOverview_ProviderLastErrAfterFailedRefresh(t *testing.T) {