GeezerVison

Description

Exposes cost data on the AI Gateway session threads endpoint (GET /api/v2/aibridge/sessions/{session_id}), so the session page can show cost alongside token usage. Backend only, the UI changes for AIGOV-600 will follow separately.

Changes

• Add cost_micros and has_unpriced_usage to AIBridgeSessionThreadsTokenUsage, populated for the session summary, each thread, and each agentic action.
• Compute the session summary cost in ListAIBridgeSessions, next to the existing session token sums, so tokens and cost come from the same rows.
• Add priced_model, the model and per-token prices used for pricing, to AIBridgeThread for the root interception (the prompt) and to each AIBridgeAgenticAction for that interception, since requests in a thread may be priced as different models. Nil when not priced.
• Update frontend mocks for the new required fields.

Related to AIGOV-600