feat(auth): add Dex JWT middleware to supervisor, routing pod, and brain MCP

Closes #6 on gitea.d-ma.be/mathias/hyperguild.

Dex is deployed at auth.d-ma.be. All three MCP servers now accept JWTs
issued by Dex in addition to static bearer tokens, enabling claude.ai
OAuth 2.0 integration without abandoning backward-compat CLI auth.

Changes:
- internal/auth/: new Validator (JWKS auto-refresh via lestrrat-go/jwx/v2),
  ProtectedResourceHandler (RFC 9728 /.well-known/oauth-protected-resource)
- internal/mcp/Server: adds optional *auth.Validator; checkAuth tries JWT
  first, then static token fallback; both-nil = auth disabled (unchanged default)
- cmd/supervisor, cmd/routing: construct Validator from DEX_ISSUER_URL +
  MCP_AUDIENCE env vars; register protected-resource handler when set
- ingestion/internal/auth/: same Validator + handler (separate module)
- ingestion/internal/mcp/BearerAuth: same JWT-or-static chain
- ingestion/cmd/server: same wiring pattern

New env vars (all optional; absent = static-token-only, same as before):
  DEX_ISSUER_URL   — Dex issuer URL (e.g. https://auth.d-ma.be)
  MCP_AUDIENCE     — expected aud claim (e.g. brain, supervisor)
  MCP_RESOURCE_URL — resource identifier for RFC 9728 metadata response

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/mcp/server_test.go

@@ -23,7 +23,7 @@ func jsonBody(t *testing.T, v any) *bytes.Buffer {
 
 func TestMCPInitialize(t *testing.T) {
 	reg := registry.New()
-	srv := mcp.NewServer(reg, "")
+	srv := mcp.NewServer(reg, "", nil)
 
 	req := httptest.NewRequest(http.MethodPost, "/mcp", jsonBody(t, map[string]any{
 		"jsonrpc": "2.0",
@@ -45,7 +45,7 @@ func TestMCPInitialize(t *testing.T) {
 
 func TestMCPToolsList(t *testing.T) {
 	reg := registry.New()
-	srv := mcp.NewServer(reg, "")
+	srv := mcp.NewServer(reg, "", nil)
 
 	req := httptest.NewRequest(http.MethodPost, "/mcp", jsonBody(t, map[string]any{
 		"jsonrpc": "2.0", "id": 2, "method": "tools/list", "params": map[string]any{},
@@ -63,7 +63,7 @@ func TestMCPToolsList(t *testing.T) {
 
 func TestMCPUnknownMethod(t *testing.T) {
 	reg := registry.New()
-	srv := mcp.NewServer(reg, "")
+	srv := mcp.NewServer(reg, "", nil)
 
 	req := httptest.NewRequest(http.MethodPost, "/mcp", jsonBody(t, map[string]any{
 		"jsonrpc": "2.0", "id": 3, "method": "unknown/method", "params": map[string]any{},
@@ -80,7 +80,7 @@ func TestMCPUnknownMethod(t *testing.T) {
 
 func TestMCPNotificationKnownMethodGetsNoResponseBody(t *testing.T) {
 	reg := registry.New()
-	srv := mcp.NewServer(reg, "")
+	srv := mcp.NewServer(reg, "", nil)
 
 	// JSON-RPC 2.0 notification: "id" field absent. Per spec, server MUST NOT
 	// reply. notifications/initialized is part of the standard MCP handshake.
@@ -116,7 +116,7 @@ func TestMCPAuth(t *testing.T) {
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {
 			reg := registry.New()
-			srv := mcp.NewServer(reg, tc.token)
+			srv := mcp.NewServer(reg, tc.token, nil)
 
 			req := httptest.NewRequest(http.MethodPost, "/mcp", jsonBody(t, map[string]any{
 				"jsonrpc": "2.0", "id": 1, "method": "initialize", "params": map[string]any{},
@@ -142,7 +142,7 @@ func TestMCPAuth(t *testing.T) {
 
 func TestMCPNotificationUnknownMethodGetsNoResponseBody(t *testing.T) {
 	reg := registry.New()
-	srv := mcp.NewServer(reg, "")
+	srv := mcp.NewServer(reg, "", nil)
 
 	req := httptest.NewRequest(http.MethodPost, "/mcp", jsonBody(t, map[string]any{
 		"jsonrpc": "2.0",