feat(adminsite): self-hosted purchase, checkout overlay, billing portal, client line-item builder

Purchase flow: name -> placeholder -> configure via the shipped PlanConfigurator
-> Paddle overlay with custom_data -> paste install UUID to link and issue.
lineItemsFor mirrors the Go catalogue.LineItems/billable exactly (base included
in exactly one place). ManageBillingButton opens the hosted portal. Paddle token
and env are baked into the build, never fetched.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

adminsite/lib/api.ts

@@ -188,6 +188,51 @@ export interface EntitlementConfig {
     features: string[];
 }
 
+export interface CheckoutOptions {
+    plans: Plan[];
+    catalogue: CatalogueRow[];
+    env: "sandbox" | "production";
+}
+
+/*
+ * lineItemsFor builds the Paddle checkout items for a configuration, client-side
+ * from the catalogue already fetched. It mirrors the Go catalogue.LineItems and
+ * its billable() exactly: base is quantity 1; the per-server unit's quantity is
+ * servers MINUS the plan's base allowance (never charge for the base — the one
+ * subtraction, kept here to match the server); a feature contributes an item
+ * only when its row has a price in this environment/term.
+ */
+export function lineItemsFor(
+    opts: CheckoutOptions,
+    choice: { tier: Tier; term: Term; servers: number; features: string[] },
+    deployment: Deployment,
+): { priceId: string; quantity: number }[] {
+    const env = opts.env;
+    const plan = opts.plans.find((p) => p.deployment === deployment && p.tier === choice.tier);
+    if (!plan) return [];
+    const rows = opts.catalogue.filter(
+        (r) => r.deployment === deployment && r.tier === choice.tier,
+    );
+    const priceOf = (r: CatalogueRow) => r.price_ids?.[env]?.[choice.term] ?? "";
+    const base = plan.base_limits.max_servers;
+    const items: { priceId: string; quantity: number }[] = [];
+    for (const r of rows) {
+        const id = priceOf(r);
+        if (r.kind === "base") {
+            if (id) items.push({ priceId: id, quantity: 1 });
+        } else if (r.kind === "limit" && r.limit_key === "max_servers") {
+            // -1 base is unlimited: nothing metered. Otherwise charge servers over base.
+            const qty = base === -1 ? 0 : choice.servers - base;
+            if (qty > 0 && id) items.push({ priceId: id, quantity: qty });
+        } else if (r.kind === "feature" && r.feature_key) {
+            if (choice.features.includes(r.feature_key) && id) {
+                items.push({ priceId: id, quantity: 1 });
+            }
+        }
+    }
+    return items;
+}
+
 export interface Entitlement {
     instance_id: string;
     account_id: string;
@@ -269,6 +314,20 @@ export const api = {
     licenseBlobUrl: (id: string) => `${API_BASE}/api/instances/${id}/license/download`,
     subscriptions: () => req<Subscription[]>("/api/subscriptions"),
 
+    entitlement: (id: string) =>
+        req<{ entitlement: Entitlement; pending: boolean }>(`/api/instances/${id}/entitlement`),
+    checkoutOptions: () => req<CheckoutOptions>("/api/checkout/options"),
+    createSelfHosted: (name: string) =>
+        post<{ instance_id: string }>("/api/instances/self-hosted", { name }),
+    updateEntitlement: (
+        id: string,
+        body: { tier: Tier; term: Term; servers: number; features: string[] },
+    ) => put<{ entitlement: Entitlement; pending: boolean }>(`/api/instances/${id}/entitlement`, body),
+    claimLink: (placeholderId: string, instance_id: string) =>
+        post<{ instance_id: string; warning?: string }>(
+            `/api/instances/${placeholderId}/claim-link`, { instance_id }),
+    billingPortal: () => post<{ url: string }>("/api/billing/portal"),
+
     accountUsers: () => req<AccountUser[]>("/api/account/users"),
     invite: (email: string, role: AccountRole) =>
         post<{ invited: boolean }>("/api/account/users", { email, role }),