feat(auth): JWT-or-static middleware + /.well-known/oauth-protected-resource (issue #5)

- internal/auth/jwt.go: JWTValidator via lestrrat-go/jwx/v2, JWKS auto-refresh
- internal/auth/bearer.go: replace Gitea PAT validation with JWT->static->default chain
- internal/gitea/client.go: always use service PAT; remove TokenFromContext lookup
- internal/config/config.go: add DexIssuerURL, MCPAudience, MCPResourceURL, StaticToken
- cmd/gitea-mcp/main.go: wire validator, fix /.well-known to return real AS list
- bearer_test.go: rewrite for new API

internal/auth/jwt.go

@@ -0,0 +1,79 @@
+package auth
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"time"
+
+	"github.com/lestrrat-go/jwx/v2/jwk"
+	"github.com/lestrrat-go/jwx/v2/jwt"
+)
+
+// JWTValidator validates bearer tokens as JWTs issued by a Dex OIDC server.
+// A nil JWTValidator always returns false — JWT validation is disabled.
+type JWTValidator struct {
+	issuer  string
+	aud     string
+	cache   *jwk.Cache
+	jwksURI string
+}
+
+// NewJWTValidator creates a validator by fetching the OIDC discovery document
+// from issuerURL. Returns nil, nil when issuerURL is empty (disabled).
+func NewJWTValidator(ctx context.Context, issuerURL, audience string) (*JWTValidator, error) {
+	if issuerURL == "" {
+		return nil, nil
+	}
+
+	resp, err := http.Get(issuerURL + "/.well-known/openid-configuration")
+	if err != nil {
+		return nil, fmt.Errorf("fetch oidc discovery: %w", err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+
+	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)
+	}
+
+	cache := jwk.NewCache(ctx)
+	if err := cache.Register(doc.JWKSURI, jwk.WithRefreshInterval(time.Hour)); err != nil {
+		return nil, fmt.Errorf("register jwks uri: %w", err)
+	}
+	// warm the cache immediately so first request doesn't block
+	if _, err := cache.Refresh(ctx, doc.JWKSURI); err != nil {
+		return nil, fmt.Errorf("warm jwks cache: %w", err)
+	}
+
+	return &JWTValidator{
+		issuer:  issuerURL,
+		aud:     audience,
+		cache:   cache,
+		jwksURI: doc.JWKSURI,
+	}, nil
+}
+
+// Validate returns true if rawToken is a valid JWT signed by the OIDC server.
+func (v *JWTValidator) Validate(ctx context.Context, rawToken string) bool {
+	if v == nil {
+		return false
+	}
+	keySet, err := v.cache.Get(ctx, v.jwksURI)
+	if err != nil {
+		return false
+	}
+	opts := []jwt.ParseOption{
+		jwt.WithKeySet(keySet),
+		jwt.WithIssuer(v.issuer),
+		jwt.WithValidate(true),
+	}
+	if v.aud != "" {
+		opts = append(opts, jwt.WithAudience(v.aud))
+	}
+	_, err = jwt.Parse([]byte(rawToken), opts...)
+	return err == nil
+}