Coverage sub-tab: implement per-provider coverage breakdowns (replace 'coming soon' stub)

[cristim]

Symptom

Inventory & Coverage > Coverage sub-tab displays a placeholder:

Per-provider coverage breakdowns are coming soon. Your overall coverage rate is on the Home dashboard.

No actual data shown. The Home dashboard's "Current Coverage" KPI is 0% for the test data, which doesn't help users diagnose where coverage is lacking.

What we need

Per-provider coverage breakdowns -- for each of AWS, Azure, GCP, display:

• Total committed usage (vCPU-hours, RDS instance-hours, etc., grouped by service)
• Total on-demand / uncovered usage in the same dimensions
• Coverage % per provider, per service
• Drilldown (or hover) showing which accounts contribute most uncovered usage

Why it matters

Users need to understand WHERE their coverage gaps are so they can target purchases. A single overall coverage % doesn't tell them whether the gap is AWS EC2, RDS, or Azure compute. Without this view, the Home page KPI is a vanity metric.

Scope notes

This is non-trivial:

• Backend: needs a new aggregation endpoint (or extension of the existing one) that joins commitment data with on-demand usage by (provider, service, account) and returns the breakdown. Likely uses the same CE / Cost Explorer + Azure Consumption / GCP Billing data already fetched for the dashboard.
• Frontend: new module for the Coverage sub-tab; table layout per provider, with totals and a coverage %, ideally sortable by gap size.
• Cross-cloud consistency: each provider exposes coverage differently (AWS has CE GetReservationCoverage / GetSavingsPlansCoverage; Azure has different views; GCP has Active Assist coverage). Must normalise.

Acceptance criteria

• Coverage sub-tab shows the per-provider breakdown when there is data.
• Empty providers show a clear empty-state ("No usage detected for AWS in this account") rather than a global "coming soon" stub.
• Refresh / period selector consistent with other views.
• Drilldown by service within provider (at minimum: compute, database, cache as separate rows).

Out of scope

• Forecasting future coverage.
• Recommending specific purchases (Opportunities page already does this).

Files likely involved

• New backend handler in internal/api/ (e.g. handler_coverage.go).
• Provider-side: existing CE coverage code; need to confirm parity for Azure and GCP.
• Frontend: new module in frontend/src/modules/coverage.ts and update to frontend/src/inventory.ts (or similar).

Source

QA exploratory testing, 2026-05-27.

[cristim]

Verified shipped: PR #760 (commit 1ba53f6, merged 2026-05-27) is in feat/multicloud-web-frontend. New handler_inventory.go endpoint + inventory.ts render per-provider coverage breakdown table, replacing the 'coming soon' stub; tests in handler_inventory_test.go and inventory.test.ts.