feat: age verification with an ID document (Didit), and closed registration

Age: a Didit provider behind the vendor-neutral age interface. A person
verifies once (ID document, passive liveness, face match) and all their
linked IMVU accounts count as adult. Webhooks are checked against Didit's
signatures; only "over 18" is kept. A later answer replaces an earlier one
(an approval taken back is revoked). Checks are limited: one unfinished
check per person, 3 in 30 days, none once verified, and a monthly cap
(DIDIT_MONTHLY_LIMIT, default 30: Didit's free plan is $10 of credit a
month and a check costs $0.30). The Account page gets an Age Verification
card.

Registration: open or closed, an admin's setting in the console
(default from REGISTRATION). Closed: new accounts can only sign in until
a moderator approves them (or declines, deleting the account); the
website shows them a waiting page and the API treats them as signed out.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/web/lib/api.ts

@@ -37,6 +37,7 @@ export type Person = Data<V1["me"]["get"]>["person"];
 export type Account = Data<V1["account"]["get"]>;
 export type Link = Account["links"][number];
 export type Challenge = Account["challenges"][number];
+export type AgeInfo = Data<V1["age"]["get"]>;
 export type OnlineEntry = Data<V1["online"]["get"]>["online"][number];
 export type ConsoleData = Data<V1["console"]["get"]>;
 export type AuditEntry = ConsoleData["audit"][number];