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>

ingestion/internal/mcp/auth.go

@@ -1,23 +1,36 @@
 package mcp
 
 import (
+	"crypto/subtle"
 	"net/http"
 	"strings"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/auth"
 )
 
-// BearerAuth returns a middleware that enforces a static bearer token on every
-// request. token must be non-empty; if it is empty, every request is rejected.
-func BearerAuth(token string, next http.Handler) http.Handler {
+// BearerAuth returns a middleware that enforces authentication on every request.
+// It tries a valid Dex JWT first (when v is non-nil), then falls back to the
+// static token. Rejects if token is empty and no valid JWT is presented.
+func BearerAuth(token string, v *auth.Validator, next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		if token == "" {
+		rawToken, ok := strings.CutPrefix(r.Header.Get("Authorization"), "Bearer ")
+		if !ok {
 			http.Error(w, "unauthorized", http.StatusUnauthorized)
 			return
 		}
-		got, ok := strings.CutPrefix(r.Header.Get("Authorization"), "Bearer ")
-		if !ok || got != token {
-			http.Error(w, "unauthorized", http.StatusUnauthorized)
+
+		if v != nil {
+			if _, err := v.Validate(r.Context(), rawToken); err == nil {
+				next.ServeHTTP(w, r)
+				return
+			}
+		}
+
+		if token != "" && subtle.ConstantTimeCompare([]byte(rawToken), []byte(token)) == 1 {
+			next.ServeHTTP(w, r)
 			return
 		}
-		next.ServeHTTP(w, r)
+
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
 	})
 }