sec(api): finish tenant-scoping sweep: inventory filter, RI-exchange quote, plans list (follow-ups to #374/#379) #384

Follow-ups from the reviews of #374 and #379 (tenant scoping)

1. internal/api/handler_inventory.go:106-119 fetchCommitmentRecords still uses the "explicit account_id bypasses allowed scope" shape. It's safe today only because the callers post-filter (handler_inventory.go:47, :197, :235). Route it through intersectAccountFilterScope (added in sec(api): intersect explicit account filters with allowed_accounts on dashboard KPIs #374), as the dashboard now does, and keep the existing sentinel short-circuit. About 6 lines; the inventory tests already stub GetAllowedAccountsAPI.
2. internal/api/handler_ri_exchange.go:1729 getExchangeQuote (POST /api/ri-exchange/quote, view:purchases) calls DescribeReservedInstances + GetReservedInstancesExchangeQuote for the ambient account with no reshapeCloudAccountInScope check, so a scoped user sees out-of-scope RI ids and quoted prices (read-only, no money moves). Add the same gate the sibling endpoints and executeExchange (fix(api): scope executeExchange to session's allowed_accounts #379) use.
3. GET /api/plans (list) applies no allowed_accounts filtering; only the by-id routes use requirePlanAccess. Filter the list by allowed_accounts.
4. Tests missing from fix(api): scope executeExchange to session's allowed_accounts #379: a restricted in-scope session (name-scoped allowed_accounts, the production shape) reaches HasPermissionForConstraintsAPI; a GetAllowedAccountsAPI error yields 500, not 403. Both behaviors are correct on main (verified with scratch tests), just not covered.

Each item needs a regression test that fails before the fix.