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/push/push.go

@@ -0,0 +1,203 @@
+// Package push keeps the household's Web Push subscriptions, one per
+// browser, keyed by the push service endpoint the browser was given, and
+// the VAPID public key browsers subscribe with.
+package push
+
+import (
+	"context"
+	"crypto/ecdh"
+	"database/sql"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"net/url"
+	"strings"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/db"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/store"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/vapid"
+)
+
+// Limits on a subscription's fields.
+const (
+	MaxEndpointBytes  = 2048
+	MaxUserAgentBytes = 512
+	p256dhSize        = 65 // uncompressed P-256 point
+	authSize          = 16
+)
+
+// keyEncoding is the unpadded base64url PushSubscription.toJSON() gives
+// the keys in.
+var keyEncoding = base64.RawURLEncoding
+
+// Subscription is a stored push subscription. Its keys are never returned:
+// only the sender needs them.
+type Subscription struct {
+	ID        string  `json:"id"`
+	Endpoint  string  `json:"endpoint"`
+	UserAgent *string `json:"user_agent"`
+	CreatedAt string  `json:"created_at"`
+	UpdatedAt string  `json:"updated_at"`
+}
+
+// SubscribeRequest is a browser's push subscription: the endpoint and keys
+// of its PushSubscription, and optionally a user agent to tell the
+// household's devices apart.
+type SubscribeRequest struct {
+	Endpoint  string
+	P256dh    string
+	Auth      string
+	UserAgent *string
+}
+
+// Problem is one invalid field of a request, named by its JSON key.
+type Problem struct {
+	Field  string
+	Reason string
+}
+
+// ValidationError lists every invalid field of a request.
+type ValidationError struct {
+	Problems []Problem
+}
+
+func (e *ValidationError) Error() string {
+	parts := make([]string, len(e.Problems))
+	for i, p := range e.Problems {
+		parts[i] = p.Field + ": " + p.Reason
+	}
+	return "invalid push subscription: " + strings.Join(parts, "; ")
+}
+
+// Service stores push subscriptions.
+type Service struct {
+	st        *store.Store
+	publicKey string
+}
+
+// NewService returns a Service over st handing out publicKey, the VAPID
+// public key in vapid's encoding.
+func NewService(st *store.Store, publicKey string) (*Service, error) {
+	if st == nil {
+		return nil, errors.New("push: nil store")
+	}
+	if _, err := vapid.ParsePublicKey(publicKey); err != nil {
+		return nil, fmt.Errorf("push: VAPID public key: %w", err)
+	}
+	return &Service{st: st, publicKey: publicKey}, nil
+}
+
+// PublicKey returns the VAPID public key, for applicationServerKey.
+func (s *Service) PublicKey() string { return s.publicKey }
+
+// Subscribe stores req by its endpoint. An endpoint already stored keeps
+// its id and created_at and has its keys and user agent replaced, so
+// subscribing twice leaves one subscription. An invalid request is a
+// *ValidationError.
+func (s *Service) Subscribe(ctx context.Context, req SubscribeRequest) (Subscription, error) {
+	var problems []Problem
+	if reason := checkEndpoint(req.Endpoint); reason != "" {
+		problems = append(problems, Problem{"endpoint", reason})
+	}
+	if reason := checkP256dh(req.P256dh); reason != "" {
+		problems = append(problems, Problem{"keys.p256dh", reason})
+	}
+	if reason := checkAuth(req.Auth); reason != "" {
+		problems = append(problems, Problem{"keys.auth", reason})
+	}
+	var ua sql.NullString
+	if req.UserAgent != nil {
+		if reason := checkUserAgent(*req.UserAgent); reason != "" {
+			problems = append(problems, Problem{"user_agent", reason})
+		}
+		ua = sql.NullString{String: *req.UserAgent, Valid: true}
+	}
+	if len(problems) > 0 {
+		return Subscription{}, &ValidationError{Problems: problems}
+	}
+
+	now := store.Now().String()
+	row, err := db.New(s.st.Writer).UpsertPushSubscription(ctx, db.UpsertPushSubscriptionParams{
+		ID:        store.NewID(),
+		Endpoint:  req.Endpoint,
+		P256dh:    req.P256dh,
+		Auth:      req.Auth,
+		UserAgent: ua,
+		CreatedAt: now,
+		UpdatedAt: now,
+	})
+	if err != nil {
+		return Subscription{}, fmt.Errorf("push: store subscription: %w", err)
+	}
+	sub := Subscription{ID: row.ID, Endpoint: row.Endpoint, CreatedAt: row.CreatedAt, UpdatedAt: row.UpdatedAt}
+	if row.UserAgent.Valid {
+		sub.UserAgent = &row.UserAgent.String
+	}
+	return sub, nil
+}
+
+// Unsubscribe removes the subscription for endpoint and reports whether
+// one was stored. An endpoint that is not stored is not an error: the
+// browser may unsubscribe after a failed delivery already removed it. An
+// invalid endpoint is a *ValidationError.
+func (s *Service) Unsubscribe(ctx context.Context, endpoint string) (bool, error) {
+	if reason := checkEndpoint(endpoint); reason != "" {
+		return false, &ValidationError{Problems: []Problem{{"endpoint", reason}}}
+	}
+	n, err := db.New(s.st.Writer).DeletePushSubscription(ctx, endpoint)
+	if err != nil {
+		return false, fmt.Errorf("push: delete subscription: %w", err)
+	}
+	return n > 0, nil
+}
+
+// checkEndpoint returns why endpoint is not a push service URL, or "".
+// Push services are always reached over https.
+func checkEndpoint(endpoint string) string {
+	if endpoint == "" {
+		return "required"
+	}
+	if len(endpoint) > MaxEndpointBytes {
+		return fmt.Sprintf("longer than %d bytes", MaxEndpointBytes)
+	}
+	u, err := url.Parse(endpoint)
+	if err != nil || u.Scheme != "https" || u.Host == "" || u.User != nil || u.Fragment != "" {
+		return "want an absolute https:// URL without credentials or fragment"
+	}
+	return ""
+}
+
+func checkP256dh(s string) string {
+	if s == "" {
+		return "required"
+	}
+	b, err := keyEncoding.DecodeString(s)
+	if err != nil || len(b) != p256dhSize {
+		return fmt.Sprintf("want the %d-byte uncompressed P-256 point in unpadded base64url", p256dhSize)
+	}
+	if _, err := ecdh.P256().NewPublicKey(b); err != nil {
+		return "not a point on P-256"
+	}
+	return ""
+}
+
+func checkAuth(s string) string {
+	if s == "" {
+		return "required"
+	}
+	b, err := keyEncoding.DecodeString(s)
+	if err != nil || len(b) != authSize {
+		return fmt.Sprintf("want %d bytes in unpadded base64url", authSize)
+	}
+	return ""
+}
+
+func checkUserAgent(s string) string {
+	switch {
+	case strings.TrimSpace(s) == "":
+		return "must not be blank; omit it instead"
+	case len(s) > MaxUserAgentBytes:
+		return fmt.Sprintf("longer than %d bytes", MaxUserAgentBytes)
+	}
+	return ""
+}