rename: "Team" billing tier to "Family"

All literal "team" tier-value references renamed to "family" across
API routes, admin UI, OpenAPI schemas, and lib/tiers.ts. The DB enum
value itself is renamed in place via ALTER TYPE ... RENAME VALUE
(migration 0044) rather than drizzle-kit's auto-generated
drop-and-recreate-the-enum migration, which would have failed against
any existing row still holding 'team' — RENAME VALUE preserves
existing data with no cast/backfill needed.

Also adds STRIPE_PLAN.md — a full Stripe billing integration plan
(Checkout+Portal, tier→Price mapping, admin billing dashboard, and a
multi-user Family-group design since Family is meant to cover several
accounts under one subscription, not one payer). Planning only, no
Stripe code yet.

v0.47.0

apps/web/app/api/v1/admin/users/route.ts

@@ -7,7 +7,7 @@ import { randomBytes, randomUUID } from "crypto";
 import { APIError } from "better-auth";
 
 const VALID_ROLES = ["user", "moderator", "admin"] as const;
-const VALID_TIERS = ["free", "pro", "team"] as const;
+const VALID_TIERS = ["free", "pro", "family"] as const;
 
 export async function POST(req: NextRequest) {
   const { session, response } = await requireAdmin();