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>

cmd/vapidgen/main.go

@@ -0,0 +1,39 @@
+// Command vapidgen prints a new VAPID key pair for Web Push. The public key
+// line goes into deploy/.env as is; the private key line goes, alone, into
+// the secret file MEALQUEST_VAPID_PRIVATE_KEY_FILE names. Both keys are
+// unpadded base64url P-256 keys, the encoding config.Load validates.
+//
+//	go run ./cmd/vapidgen
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/config"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/vapid"
+)
+
+func main() {
+	if len(os.Args) > 1 {
+		fmt.Fprintln(os.Stderr, "usage: vapidgen (takes no arguments)")
+		os.Exit(2)
+	}
+	if err := run(os.Stdout); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
+
+// run writes a new key pair to w in four lines: a comment and the public
+// key's environment line, then a comment and the private key alone.
+func run(w io.Writer) error {
+	pair, err := vapid.Generate()
+	if err != nil {
+		return err
+	}
+	_, err = fmt.Fprintf(w, "# Environment (deploy/.env):\n%s=%s\n# Private key: this next line alone is the file %s names.\n%s\n",
+		config.EnvVAPIDPublicKey, pair.PublicKey, config.EnvVAPIDPrivateKeyFile, pair.PrivateKey)
+	return err
+}