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/cmd/server/main.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/mathiasbq/hyperguild/ingestion/internal/api"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/auth"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/llm"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/mcp"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/pipeline"
@@ -80,7 +81,25 @@ func main() {
 	mux.HandleFunc("POST /ingest-raw", h.IngestRaw)
 	mux.HandleFunc("POST /backfill-refs", h.BackfillRefs)
 	mux.HandleFunc("GET /pass-rate", h.PassRate)
-	mux.Handle("/mcp", mcp.BearerAuth(mcpToken, mcpSrv))
+	var jwtValidator *auth.Validator
+	if dexURL := os.Getenv("DEX_ISSUER_URL"); dexURL != "" {
+		audience := os.Getenv("MCP_AUDIENCE")
+		v, err := auth.NewValidator(dexURL, audience)
+		if err != nil {
+			logger.Error("build jwt validator", "err", err)
+			os.Exit(1)
+		}
+		jwtValidator = v
+		logger.Info("jwt auth enabled", "issuer", dexURL)
+	}
+
+	mux.Handle("/mcp", mcp.BearerAuth(mcpToken, jwtValidator, mcpSrv))
+
+	if dexURL := os.Getenv("DEX_ISSUER_URL"); dexURL != "" {
+		resourceURL := os.Getenv("MCP_RESOURCE_URL")
+		mux.HandleFunc("GET /.well-known/oauth-protected-resource",
+			auth.ProtectedResourceHandler(resourceURL, os.Getenv("DEX_ISSUER_URL")))
+	}
 
 	addr := ":" + port
 	watchIntervalLog := "disabled"