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/auth/jwt.go

@@ -0,0 +1,84 @@
+package auth
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"time"
+
+	"github.com/lestrrat-go/jwx/v2/jwk"
+	"github.com/lestrrat-go/jwx/v2/jwt"
+)
+
+// Validator validates Bearer JWTs issued by a Dex (OIDC) authorization server.
+// Audience is optional; leave empty to skip audience validation.
+type Validator struct {
+	issuer   string
+	audience string
+	jwksURI  string
+	cache    *jwk.Cache
+}
+
+// NewValidator fetches the OIDC discovery document from issuerURL, extracts
+// jwks_uri, seeds the JWKS cache, and returns a ready Validator.
+// If DEX_ISSUER_URL is not set the caller should pass "" and skip construction.
+func NewValidator(issuerURL, audience string) (*Validator, error) {
+	resp, err := http.Get(issuerURL + "/.well-known/openid-configuration") //nolint:noctx
+	if err != nil {
+		return nil, fmt.Errorf("fetch oidc discovery: %w", err)
+	}
+	defer resp.Body.Close() //nolint:errcheck
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("oidc discovery: status %d", resp.StatusCode)
+	}
+
+	var doc struct {
+		JWKSURI string `json:"jwks_uri"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&doc); err != nil {
+		return nil, fmt.Errorf("decode oidc discovery: %w", err)
+	}
+	if doc.JWKSURI == "" {
+		return nil, fmt.Errorf("oidc discovery: empty jwks_uri")
+	}
+
+	ctx := context.Background()
+	cache := jwk.NewCache(ctx)
+	if err := cache.Register(doc.JWKSURI, jwk.WithMinRefreshInterval(time.Hour)); err != nil {
+		return nil, fmt.Errorf("register jwks cache: %w", err)
+	}
+	if _, err := cache.Refresh(ctx, doc.JWKSURI); err != nil {
+		return nil, fmt.Errorf("initial jwks fetch: %w", err)
+	}
+
+	return &Validator{
+		issuer:   issuerURL,
+		audience: audience,
+		jwksURI:  doc.JWKSURI,
+		cache:    cache,
+	}, nil
+}
+
+// Validate parses and validates rawToken. Returns the subject claim on success.
+func (v *Validator) Validate(ctx context.Context, rawToken string) (string, error) {
+	keySet, err := v.cache.Get(ctx, v.jwksURI)
+	if err != nil {
+		return "", fmt.Errorf("get jwks: %w", err)
+	}
+
+	opts := []jwt.ParseOption{
+		jwt.WithKeySet(keySet),
+		jwt.WithValidate(true),
+		jwt.WithIssuer(v.issuer),
+	}
+	if v.audience != "" {
+		opts = append(opts, jwt.WithAudience(v.audience))
+	}
+
+	tok, err := jwt.ParseString(rawToken, opts...)
+	if err != nil {
+		return "", fmt.Errorf("validate jwt: %w", err)
+	}
+	return tok.Subject(), nil
+}