feat(cli): tapir invite <email> + TAPIR_PUBLIC_URL config

Mints a single-use invitation and prints the absolute claim URL for the
operator to send. The URL base is TAPIR_PUBLIC_URL (default
https://tapir.d-ma.be). runInvite is factored from config/store wiring so
it's unit-tested against a fake inviter — no Postgres.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/tapir/main.go

@@ -53,6 +53,8 @@ func main() {
 		err = cmdRun(ctx, log)
 	case "serve":
 		err = cmdServe(ctx, log)
+	case "invite":
+		err = cmdInvite(ctx, os.Args[2:])
 	default:
 		usage()
 		os.Exit(2)
@@ -71,6 +73,7 @@ usage:
   tapir auth              one-time: authorize YouTube and store a refresh token
   tapir run               detect new videos, summarize, deliver to your store
   tapir serve             run the web UI (read summaries, record watch/skip/save)
+  tapir invite <email>    mint an invitation link for a new user (host-side)
   tapir list [-limit N]   list stored summaries, recent first
   tapir show <video-id>   show one summary in full