[Epic]: A governed observability surface — one dashboard catalogue, Copilot reporting, trace-log correlation

### Summary

One dashboard catalogue that governs both what is kept and what is added, plus the two remaining correlation and datasource gaps.

### Intent

The dashboard set has accreted to 20+ generated modules on `main` while a separate workstream proposes four more, and nothing says which are sanctioned. A pruning ticket and an addition ticket that do not know about each other produce the next accretion — so they belong under one catalogue.

### Children

- **#530** — decide and enforce the sanctioned dashboard set
- **#674** — traces-to-logs correlation
- **#879** — read-only Grafana Postgres datasource over the governance database
- **#880** — four Copilot governance dashboards and five ingestion alerts

### Acceptance Criteria

1. A catalogue at `docs/observability/dashboard-catalogue.md` lists every kept dashboard and its panel spec.
2. **CI fails when `tools/dashboards/src/dashboards/` contains a module absent from the catalogue** — a new dashboard lands by adding a catalogue row, so the set cannot silently grow again.
3. `uv run dashboards check` passes.

### Out of Scope

- Grafana provisioning mechanics and datasource plumbing beyond what #879 needs.

### Technical Context

⚠️ **#879's premise is stale**: it says "no governance database exists yet", but the `governance` role and Database are on `main` in `charts/lightbridge-db/values.yaml`, and its dependency `lightbridge-governance#12` is closed.

⚠️ **#879 is building a datasource over a database ADR-0014 decommissions.** `lightbridge-governance` consolidated its telemetry store into the authz usage store on 2026-08-31; that ADR's sweep did not reach this repo. Settle the target before building the grant — see `lightbridge-governance#197`.

⚠️ #530's current AC says "every **unplanned** dashboard is removed", measured against a local planning file not in the repo. AC 2 above replaces that with something re-readable.

### AI Usage Declaration

Grouping identified and drafted by Claude Opus 5 during the 2026-09-01 triage sweep; #879's stale premise and the ADR-0014 collision were verified against `origin/main`. AI output is not truth; the owner verifies.