[Ticket]: Observability — per-project attribution, dashboards & budget-burn alerts

### Type

Feature

Parent epic: #531. Intended labels: `ticket`, `observability`.

### Summary

Carry `x-project-id` through the attribution pipeline: Envoy access-log field → Alloy label promotion (ADR-0046 flatten stage) → `project_id` label on the ADR-0058 Mimir counters (`gen_ai_usage_cost_micro_usd`, `gen_ai_usage_tokens`, `gen_ai_requests`) and Loki streams. Add per-project Grafana panels (spend, tokens, top members, envelope burn-down) and ADR-0059 alert rules at 80% of project envelope.

### Intent

Leads see their project's consumption; governance sees all projects; the audit unit from the epic ("show me everything project X did") becomes a label filter.

### Acceptance Criteria

- [ ] `project_id` promoted as a label (bounded cardinality — projects, not members; per ADR-0011 discipline).
- [ ] Existing per-user dashboards unaffected; scoreboard (ADR-0060) gains a project dimension where cheap.
- [ ] Per-project dashboard generated via `tools/dashboards` (ADR-0008), shipped as a `GrafanaDashboard` CR.
- [ ] Alert rule: project envelope 80% burn → Discord (ADR-0059 contact point).
- [ ] Loki/Mimir query contract documented (label names, absent-value convention).

### Human accountable owner

@Koufan-De-King

### AI Usage Declaration

- [x] Drafting the ticket
- [x] I have declared AI usage above (ticked the relevant items, or "Not used").