P17: Add per-user and per-host tracing for daemon usage

[wauputr4]

Goal

Extend usage tracing so self-hosted operators can see which users/API keys consumed their daemon capacity.

User Story

As a self-hosted host, I can see per-user and per-API-key usage routed through my daemon node, including model, token usage, status, latency, and timestamps. As the Mizan operator, I can enforce limits and audit requests without exposing raw provider secrets.

Scope

• Extend usage/request log model with daemon host/node references.
• Add host-facing usage query endpoints or admin endpoints filtered by daemon node/host.
• Ensure existing user-facing usage remains scoped to the requesting user.
• Add metrics labels for daemon node/host where safe.
• Avoid exposing conversation content in logs by default.

Acceptance Criteria

• Every daemon-backed completed request records user id, API key id, node id, route id, model, status, latency, and usage tokens.
• Host/admin can query daemon usage by node and date range.
• User can query their own usage without seeing host-private metadata beyond public model/route info.
• Metrics include safe daemon/node dimensions with cardinality guardrails.
• Tests cover per-user and per-host usage isolation.