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/server/push.go

@@ -0,0 +1,98 @@
+package server
+
+import (
+	"errors"
+	"log/slog"
+	"net/http"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/push"
+)
+
+// maxPushBody bounds a POST or DELETE /push/subscriptions body: an endpoint
+// of push.MaxEndpointBytes, the keys and a user agent of
+// push.MaxUserAgentBytes, with room for JSON escaping.
+const maxPushBody = 8 << 10
+
+type vapidPublicKeyResponse struct {
+	PublicKey string `json:"public_key"`
+}
+
+// pushSubscriptionBody is the POST /push/subscriptions body: the
+// endpoint and keys of PushSubscription.toJSON(), without its
+// expirationTime, and an optional user_agent.
+type pushSubscriptionBody struct {
+	Endpoint  string               `json:"endpoint"`
+	Keys      pushSubscriptionKeys `json:"keys"`
+	UserAgent *string              `json:"user_agent"`
+}
+
+type pushSubscriptionKeys struct {
+	P256dh string `json:"p256dh"`
+	Auth   string `json:"auth"`
+}
+
+// pushUnsubscribeBody is the DELETE /push/subscriptions body.
+type pushUnsubscribeBody struct {
+	Endpoint string `json:"endpoint"`
+}
+
+// getVAPIDPublicKey answers GET /push/vapid-public-key with the key
+// browsers pass as applicationServerKey. The private key is never served.
+func getVAPIDPublicKey(logger *slog.Logger, svc *push.Service) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		writeJSON(w, r, logger, http.StatusOK, vapidPublicKeyResponse{PublicKey: svc.PublicKey()})
+	}
+}
+
+// postPushSubscription stores the browser's subscription by its endpoint
+// and answers 200 with it, without its keys. Posting an endpoint already
+// stored updates it.
+func postPushSubscription(logger *slog.Logger, svc *push.Service) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		body, err := decodePatch[pushSubscriptionBody](http.MaxBytesReader(w, r.Body, maxPushBody))
+		if err != nil {
+			writeError(w, r, logger, http.StatusBadRequest, "invalid_request", err.Error())
+			return
+		}
+		sub, err := svc.Subscribe(r.Context(), push.SubscribeRequest{
+			Endpoint:  body.Endpoint,
+			P256dh:    body.Keys.P256dh,
+			Auth:      body.Keys.Auth,
+			UserAgent: body.UserAgent,
+		})
+		if err != nil {
+			writePushError(w, r, logger, "store push subscription", err)
+			return
+		}
+		writeJSON(w, r, logger, http.StatusOK, sub)
+	}
+}
+
+// deletePushSubscription removes the subscription for the body's endpoint
+// and answers 204, whether or not it was stored.
+func deletePushSubscription(logger *slog.Logger, svc *push.Service) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		body, err := decodePatch[pushUnsubscribeBody](http.MaxBytesReader(w, r.Body, maxPushBody))
+		if err != nil {
+			writeError(w, r, logger, http.StatusBadRequest, "invalid_request", err.Error())
+			return
+		}
+		if _, err := svc.Unsubscribe(r.Context(), body.Endpoint); err != nil {
+			writePushError(w, r, logger, "delete push subscription", err)
+			return
+		}
+		w.WriteHeader(http.StatusNoContent)
+	}
+}
+
+// writePushError maps a push.Service error to its API error: an invalid
+// subscription is 400 invalid_subscription, anything else 500.
+func writePushError(w http.ResponseWriter, r *http.Request, logger *slog.Logger, action string, err error) {
+	var invalid *push.ValidationError
+	if errors.As(err, &invalid) {
+		writeError(w, r, logger, http.StatusBadRequest, "invalid_subscription", invalid.Error())
+		return
+	}
+	logger.ErrorContext(r.Context(), action, "error", err)
+	writeError(w, r, logger, http.StatusInternalServerError, "internal", "internal error")
+}