feat(paperless-ai): mint the LLM token with a machine key, not an init container

The init container fetched a token once at pod start with a client_credentials grant
against a web app. A web app has no such grant, so the fetch could not work; and a
token fetched once is dead within the hour while the pod runs for weeks.

llm-token-vendor runs beside the app. It signs a machine-key assertion for the
paperless-llm service account, exchanges it for a token, and rewrites
/app/data/llm_token.txt atomically at 80% of the token lifetime. The vendor image
keeps using CUSTOM_API_KEY_FILE, so it needs no change and no OIDC support.

The key is re-read on every mint, so a rotation lands without a restart: publish the
new key, the next mint uses it, retire the old one.

paperless-llm is a separate service account from poimen-llm, so revoking one caller
leaves the other working.

k8s/apps/llm-token-vendor/token_vendor_test.go

@@ -0,0 +1,64 @@
+package main
+
+import (
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/base64"
+	"encoding/json"
+	"encoding/pem"
+	"strings"
+	"testing"
+	"time"
+)
+
+func key(t *testing.T) []byte {
+	t.Helper()
+	k, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		t.Fatalf("cannot generate a key: %v", err)
+	}
+	return pem.EncodeToMemory(&pem.Block{Type: "RSA PRIVATE KEY", Bytes: x509.MarshalPKCS1PrivateKey(k)})
+}
+
+func seg(t *testing.T, s string, out any) {
+	t.Helper()
+	raw, err := base64.RawURLEncoding.DecodeString(s)
+	if err != nil {
+		t.Fatalf("not base64url: %v", err)
+	}
+	if err := json.Unmarshal(raw, out); err != nil {
+		t.Fatalf("not JSON: %v", err)
+	}
+}
+
+func TestAssertionCarriesTheKeyAndTheAudience(t *testing.T) {
+	c := config{issuer: "https://zitadel.example", userID: "3970", keyID: "3971"}
+	tok, err := assertion(c, time.Unix(1_700_000_000, 0), key(t))
+	if err != nil {
+		t.Fatalf("signing failed: %v", err)
+	}
+	parts := strings.Split(tok, ".")
+	if len(parts) != 3 {
+		t.Fatalf("a JWT has three segments, got %d", len(parts))
+	}
+	var h map[string]any
+	seg(t, parts[0], &h)
+	if h["kid"] != "3971" || h["alg"] != "RS256" {
+		t.Fatalf("header must name the key and the algorithm, got %v", h)
+	}
+	var cl map[string]any
+	seg(t, parts[1], &cl)
+	if cl["iss"] != "3970" || cl["sub"] != "3970" || cl["aud"] != "https://zitadel.example" {
+		t.Fatalf("claims are wrong: %v", cl)
+	}
+	if int64(cl["iat"].(float64)) != 1_699_999_995 {
+		t.Fatalf("iat must be backdated five seconds, got %v", cl["iat"])
+	}
+}
+
+func TestAssertionRejectsAKeyThatIsNotPEM(t *testing.T) {
+	if _, err := assertion(config{userID: "u", issuer: "i"}, time.Now(), []byte("nope")); err == nil {
+		t.Fatalf("a non-PEM key must be rejected")
+	}
+}