feat(cloud): Enforce per-plan AI token allowance

[nfebe]

Task

Back the usage half of Entitlements (remaining / consume) with per-plan AI token allowances.

Actions

• Add a token_allowance and a monthly reset period to each plan.
• Create an ai_usage_counters table (user_id, period_start, tokens_used); this storage lives in the cloud plugin, never core.
• Implement remaining('ai_tokens') as allowance minus current-period usage, and consume('ai_tokens', n) as an idempotent increment keyed by chat message id, inside the cloud Entitlements implementation.

Acceptance

• A user over their allowance gets a quota-exceeded response from POST /api/v1/ai/chat.
• The counter equals the summed usage stored in ChatMessage.result['usage'] and rolls over monthly.
• A deployment with the cloud plugin disabled stays unlimited.

[kofimokome]

@nfebe how do I get the user's current subscription?

[kofimokome]

Also the consume method takes just the key and number of tokens.
It's dificult to get the particular chat id from the cloud plugin, except we add a third parameter to the consume method