feat: add invite code system and multi-image Drone pipeline

- Add InviteCode model with code, maxUses, expiry, usage tracking
- Registration now requires a valid invite code
- New users get USER role instead of ADMIN
- Admin-only /invites page to create, manage, and share invite codes
- Invite links auto-fill code via ?code= URL param
- Drone pipeline now builds app, worker, and bot images separately
- Add NEXT_PUBLIC_APP_URL build arg to fix URL redirects

prisma/migrations/20260321140000_add_invite_codes/migration.sql

@@ -0,0 +1,21 @@
+-- CreateTable
+CREATE TABLE "invite_codes" (
+    "id" TEXT NOT NULL,
+    "code" VARCHAR(32) NOT NULL,
+    "maxUses" INTEGER NOT NULL DEFAULT 1,
+    "uses" INTEGER NOT NULL DEFAULT 0,
+    "expiresAt" TIMESTAMP(3),
+    "createdBy" TEXT NOT NULL,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "invite_codes_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "invite_codes_code_key" ON "invite_codes"("code");
+
+-- CreateIndex
+CREATE INDEX "invite_codes_code_idx" ON "invite_codes"("code");
+
+-- AddForeignKey
+ALTER TABLE "invite_codes" ADD CONSTRAINT "invite_codes_createdBy_fkey" FOREIGN KEY ("createdBy") REFERENCES "User"("id") ON DELETE CASCADE ON UPDATE CASCADE;