feat(auth): trust the k8s cluster OIDC issuer for ServiceAccount tokens (ADR-0011 D1)

Additive: gitea-mcp now validates JWTs from a LIST of issuers — the existing
Authentik issuer (DEX_ISSUER_URL) AND, when K8S_ISSUER_URL is set, the in-cluster
k8s OIDC issuer for audience-bound ServiceAccount tokens. Lets in-cluster pods
authenticate with kubelet-rotated projected SA tokens instead of a static bearer.

- config: K8S_ISSUER_URL + K8S_MCP_AUDIENCE.
- cmd/gitea-mcp/k8soidc.go: HTTP client that fetches the k8s OIDC discovery/JWKS
  with the cluster CA + this pod's SA bearer (k3s requires an authed fetch;
  anonymous is 401).
- main.go: build the issuer list; switch NewJWTValidator -> NewMultiJWTValidator.
  The k8s issuer is best-effort — if its client can't be built (not in a pod) or
  its discovery is unreachable at startup, it is DROPPED and we fall back so
  Authentik/static auth is never taken down. Smoke-tested: off-pod it logs the
  skip and starts static-only; static-bearer /mcp returns 400 (auth passed), not 401.
- bump mcp-chassis v0.3.0 -> v0.5.0 (multi-issuer + per-issuer HTTPClient).

Refs infra ADR-0011; enables retiring the in-cluster static bearer.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/gitea-mcp/k8soidc.go

@@ -0,0 +1,55 @@
+package main
+
+import (
+	"crypto/tls"
+	"crypto/x509"
+	"fmt"
+	"net/http"
+	"os"
+	"strings"
+	"time"
+)
+
+const (
+	saCAFile    = "/var/run/secrets/kubernetes.io/serviceaccount/ca.crt"
+	saTokenFile = "/var/run/secrets/kubernetes.io/serviceaccount/token" //nolint:gosec // path, not a secret
+)
+
+// k8sBearerRT adds this pod's ServiceAccount bearer to every request. k3s serves
+// its OIDC discovery/JWKS over the cluster CA and requires an AUTHENTICATED
+// request (anonymous is 401), so the JWKS fetch must carry a token — ADR-0011.
+type k8sBearerRT struct {
+	token string
+	base  http.RoundTripper
+}
+
+func (b k8sBearerRT) RoundTrip(r *http.Request) (*http.Response, error) {
+	r.Header.Set("Authorization", "Bearer "+b.token)
+	return b.base.RoundTrip(r)
+}
+
+// k8sOIDCClient builds an HTTP client that can reach the in-cluster k8s OIDC
+// discovery + JWKS: it trusts the cluster CA and carries this pod's SA bearer.
+// Returns an error (not running in a pod, files unreadable) so the caller can
+// skip the k8s issuer without disabling other auth.
+func k8sOIDCClient() (*http.Client, error) {
+	ca, err := os.ReadFile(saCAFile)
+	if err != nil {
+		return nil, fmt.Errorf("read cluster CA: %w", err)
+	}
+	pool := x509.NewCertPool()
+	if !pool.AppendCertsFromPEM(ca) {
+		return nil, fmt.Errorf("parse cluster CA: no certs in %s", saCAFile)
+	}
+	tok, err := os.ReadFile(saTokenFile)
+	if err != nil {
+		return nil, fmt.Errorf("read SA token: %w", err)
+	}
+	return &http.Client{
+		Timeout: 15 * time.Second,
+		Transport: k8sBearerRT{
+			token: strings.TrimSpace(string(tok)),
+			base:  &http.Transport{TLSClientConfig: &tls.Config{RootCAs: pool, MinVersion: tls.VersionTLS12}},
+		},
+	}, nil
+}

cmd/gitea-mcp/main.go

@@ -37,7 +37,36 @@ func main() {
 
 	ctx := context.Background()
 
-	jwtValidator, jwtInitErr := chassisauth.NewJWTValidator(ctx, cfg.DexIssuerURL, cfg.MCPAudience)
+	// Build the trusted-issuer list. Authentik (DEX_ISSUER_URL) for interactive/
+	// web clients; the in-cluster k8s OIDC issuer for ServiceAccount tokens
+	// (ADR-0011 D1). The k8s issuer is ADDITIVE and best-effort: if its client
+	// can't be built (not in a pod) or its discovery is unreachable at startup,
+	// it is dropped so existing Authentik/static auth is never taken down.
+	var issuers []chassisauth.IssuerConfig
+	if cfg.DexIssuerURL != "" {
+		issuers = append(issuers, chassisauth.IssuerConfig{IssuerURL: cfg.DexIssuerURL, Audience: cfg.MCPAudience})
+	}
+	if cfg.K8sIssuerURL != "" {
+		if client, cerr := k8sOIDCClient(); cerr != nil {
+			logger.Warn("k8s OIDC client init failed; SA-token auth disabled (other auth unaffected)", "err", cerr)
+		} else {
+			issuers = append(issuers, chassisauth.IssuerConfig{IssuerURL: cfg.K8sIssuerURL, Audience: cfg.K8sAudience, HTTPClient: client})
+		}
+	}
+
+	jwtValidator, jwtInitErr := chassisauth.NewMultiJWTValidator(ctx, issuers)
+	if jwtInitErr != nil && cfg.K8sIssuerURL != "" && len(issuers) > 1 {
+		// A configured issuer (likely the in-cluster k8s OIDC) was unreachable at
+		// startup. Don't let it take down Authentik JWT auth — retry without it.
+		logger.Warn("multi-issuer init failed; retrying without the k8s issuer", "err", jwtInitErr)
+		pub := make([]chassisauth.IssuerConfig, 0, len(issuers))
+		for _, ic := range issuers {
+			if ic.IssuerURL != cfg.K8sIssuerURL {
+				pub = append(pub, ic)
+			}
+		}
+		jwtValidator, jwtInitErr = chassisauth.NewMultiJWTValidator(ctx, pub)
+	}
 	if jwtInitErr != nil {
 		logger.Warn("jwt validator init failed; JWT auth degraded", "err", jwtInitErr)
 	}