Record a spend cap refusal as a security event #2636

Part of #2422 (F-DSK). Found by an independent review of #2590.

The model gateway refuses a call over the LLM spend cap (ModelErrorCodes llm_spend_cap_exceeded, in ModelGateway.ReserveAsync), but appends no audit event. SEC-08 asks for every cap exceeded on the security events view, so this one can never reach /security or its header badge.

Scope

• A refused call over the spend cap appends an audit event, once per refusal, naming the profile and the cap.
• SecurityEvents.Operations includes it, so it is listed on /security and counted in the badge.
• Tests: a call over the cap appends the event, and it shows on /security.

Out of scope

• Injection flagged, which lands with its own feature.

Acceptance

• A spend cap refusal is listed on /security with the agent or person whose call it was.