feat(usage): let chat callers see their own quota and conversation cost

[albanm]

Any chat caller (org member, external user, anonymous visitor, account owner) can now see their own AI quota and what the current conversation has cost.

• GET /api/gateway/:type/:id/usage returns the caller's daily/weekly/monthly windows (used, limit, reset date), resolved with the same identity as a completion. The org credit cap and the anonymous+external pool are reported as a status (ok/exhausted + reset date); their numbers are only shown to admins of the account.
• Gateway completions report usage.cost in credits (OpenRouter's convention), including the moderation check when its verdict settles before the gate opens.
• The chat sums those costs into a conversation total, shown with the quota windows in a new Consumption tab of the chat settings dialog.
• Quota 429s carry period and resets_at, drop the shared budgets' numbers for non-admins, are no longer retried client-side, and render as a localized message ("Your daily AI quota is used up. It resets on …").
• Fix: recordUsage without a userId (owner of a user account) could increment an arbitrary usage document of that owner, e.g. an external user's record.
• Dev fixtures: test1-user1/dev1-user1 had the non-existent org role user1; now user.

Why: a user should be able to see their own quotas and consumption, including the current conversation's.

Heads-up:

• 429 body change: for account/untrusted scopes, usage/limit are now omitted unless the caller is an admin; period is added.
• Org members can still read the credit cap through GET /api/limits/:type/:id (unchanged, intentional), so the redaction mainly protects shared budgets from external/anonymous callers.
• The streamed gateway path now awaits the moderation gate before writing the finish/usage chunk (it already did before [DONE]).
• The conversation total is informational: a moderation verdict landing after the gate failed open, or a blocked request, is recorded server-side but not reported to the client.