Add VAPID config and the push subscription API

Require MEALQUEST_VAPID_PUBLIC_KEY, MEALQUEST_VAPID_PRIVATE_KEY_FILE and
MEALQUEST_VAPID_SUBJECT at boot. Both keys are unpadded base64url P-256
keys, the encoding webpush-go reads; boot derives the public point from the
private key and refuses a pair that does not match. The subject must be
mailto:<address> or an https:// URL.

internal/push stores subscriptions by endpoint: GET
/api/v1/push/vapid-public-key serves only the public key, POST
/api/v1/push/subscriptions upserts {endpoint, keys.p256dh, keys.auth,
user_agent}, and DELETE removes by endpoint, answering 204 whether or not
it was stored. cmd/vapidgen prints a new key pair. deploy/compose.yaml
mounts the vapid_private_key secret.

Maestro task e8a7b1f4-327a-478b-88c7-5d265afda153.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/vapid/vapid.go

@@ -0,0 +1,87 @@
+// Package vapid generates and validates the VAPID key pair Web Push signs
+// with (RFC 8292): a P-256 (ES256) key pair in the encoding webpush-go
+// reads. Both keys are unpadded base64url: the public key is the 65-byte
+// uncompressed point and the private key the 32-byte scalar.
+package vapid
+
+import (
+	"crypto/ecdh"
+	"crypto/rand"
+	"encoding/base64"
+	"errors"
+	"fmt"
+)
+
+// encoding is unpadded base64url. Strict refuses non-zero trailing bits, so
+// every key has exactly one accepted spelling.
+var encoding = base64.RawURLEncoding.Strict()
+
+const (
+	publicKeySize  = 65 // 0x04 || X || Y
+	privateKeySize = 32
+)
+
+// KeyPair is a VAPID key pair in its stored encoding.
+type KeyPair struct {
+	PublicKey  string
+	PrivateKey string
+}
+
+// Generate returns a new random key pair.
+func Generate() (KeyPair, error) {
+	k, err := ecdh.P256().GenerateKey(rand.Reader)
+	if err != nil {
+		return KeyPair{}, fmt.Errorf("vapid: generate key: %w", err)
+	}
+	return KeyPair{
+		PublicKey:  encoding.EncodeToString(k.PublicKey().Bytes()),
+		PrivateKey: encoding.EncodeToString(k.Bytes()),
+	}, nil
+}
+
+// ParsePublicKey parses an encoded public key, which must be a point on
+// P-256.
+func ParsePublicKey(s string) (*ecdh.PublicKey, error) {
+	b, err := encoding.DecodeString(s)
+	if err != nil {
+		return nil, errors.New("not unpadded base64url")
+	}
+	if len(b) != publicKeySize || b[0] != 0x04 {
+		return nil, fmt.Errorf("want the %d-byte uncompressed P-256 point, got %d bytes", publicKeySize, len(b))
+	}
+	k, err := ecdh.P256().NewPublicKey(b)
+	if err != nil {
+		return nil, errors.New("not a point on P-256")
+	}
+	return k, nil
+}
+
+// ParsePrivateKey parses an encoded private key. Its errors never quote the
+// key.
+func ParsePrivateKey(s string) (*ecdh.PrivateKey, error) {
+	b, err := encoding.DecodeString(s)
+	if err != nil {
+		return nil, errors.New("not unpadded base64url")
+	}
+	if len(b) != privateKeySize {
+		return nil, fmt.Errorf("want a %d-byte P-256 scalar, got %d bytes", privateKeySize, len(b))
+	}
+	k, err := ecdh.P256().NewPrivateKey(b)
+	if err != nil {
+		return nil, errors.New("not a valid P-256 scalar")
+	}
+	return k, nil
+}
+
+// ErrMismatch is returned by CheckPair when the private key's public key is
+// not the given public key.
+var ErrMismatch = errors.New("private key does not belong to the public key")
+
+// CheckPair reports whether private is the private key of public, by
+// deriving the public point from private and comparing.
+func CheckPair(public *ecdh.PublicKey, private *ecdh.PrivateKey) error {
+	if !private.PublicKey().Equal(public) {
+		return ErrMismatch
+	}
+	return nil
+}

internal/vapid/vapid_test.go

@@ -0,0 +1,92 @@
+package vapid
+
+import (
+	"encoding/base64"
+	"strings"
+	"testing"
+)
+
+func TestGenerateRoundTrips(t *testing.T) {
+	t.Parallel()
+	a, err := Generate()
+	if err != nil {
+		t.Fatal(err)
+	}
+	b, err := Generate()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if a == b {
+		t.Fatal("two generated key pairs are equal")
+	}
+	// 65 and 32 bytes in unpadded base64url.
+	if len(a.PublicKey) != 87 || len(a.PrivateKey) != 43 {
+		t.Errorf("key lengths = %d, %d; want 87, 43", len(a.PublicKey), len(a.PrivateKey))
+	}
+	pub, err := ParsePublicKey(a.PublicKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	priv, err := ParsePrivateKey(a.PrivateKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := CheckPair(pub, priv); err != nil {
+		t.Errorf("CheckPair of a generated pair: %v", err)
+	}
+	other, err := ParsePrivateKey(b.PrivateKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := CheckPair(pub, other); err != ErrMismatch {
+		t.Errorf("CheckPair of mismatched keys = %v, want ErrMismatch", err)
+	}
+}
+
+func TestParseRejectsMalformedKeys(t *testing.T) {
+	t.Parallel()
+	pair, err := Generate()
+	if err != nil {
+		t.Fatal(err)
+	}
+	raw := func(b []byte) string { return base64.RawURLEncoding.EncodeToString(b) }
+	compressed, _ := base64.RawURLEncoding.DecodeString(pair.PublicKey)
+	compressed = append([]byte{0x02}, compressed[1:33]...)
+	offCurve := make([]byte, 65)
+	offCurve[0] = 0x04
+	offCurve[64] = 1
+	order := []byte{ // the P-256 group order n, not a valid scalar
+		0xff, 0xff, 0xff, 0xff, 0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+		0xbc, 0xe6, 0xfa, 0xad, 0xa7, 0x17, 0x9e, 0x84, 0xf3, 0xb9, 0xca, 0xc2, 0xfc, 0x63, 0x25, 0x51,
+	}
+
+	for name, s := range map[string]string{
+		"empty":          "",
+		"padded":         pair.PublicKey + "=",
+		"standard alpha": strings.NewReplacer("-", "+", "_", "/").Replace(pair.PublicKey) + "+/",
+		"compressed":     raw(compressed),
+		"off curve":      raw(offCurve),
+		"private key":    pair.PrivateKey,
+	} {
+		if _, err := ParsePublicKey(s); err == nil {
+			t.Errorf("ParsePublicKey(%s) succeeded", name)
+		}
+	}
+	for name, s := range map[string]string{
+		"empty":      "",
+		"padded":     pair.PrivateKey + "=",
+		"PEM":        "-----BEGIN EC PRIVATE KEY-----",
+		"zero":       raw(make([]byte, 32)),
+		"group size": raw(order),
+		"public key": pair.PublicKey,
+	} {
+		_, err := ParsePrivateKey(s)
+		if err == nil {
+			t.Errorf("ParsePrivateKey(%s) succeeded", name)
+			continue
+		}
+		if s != "" && strings.Contains(err.Error(), s) {
+			t.Errorf("ParsePrivateKey(%s) error quotes the key: %v", name, err)
+		}
+	}
+}