Cost tracking + Profit/Loss reporting (#82)

[v0l]

Closes #82.

Summary

Adds optional, admin-only cost tracking used to compute profit/loss, plus a P/L report endpoint.

Rather than adding cost columns to vm_host/ip_range, costs live in one generic resource_cost table weakly linked to any resource via (resource_type, resource_id). This is extensible (new cost-bearing resource kinds need no schema change) and lets a single resource carry multiple cost rows (e.g. a host's recurring rent and a one-time hardware investment).

DB changes

• New resource_cost table: resource_type (TINYINT enum: vm_host/ip_range), resource_id, cost_type (recurring/one_time), amount (smallest units; per-IP for ip_range recurring), currency, interval_amount/interval_type (reuses existing IntervalType), billing_start, billing_end (null = ongoing), timestamps.
• AdminResource::ResourceCost (24) + migration granting super_admin the full permission set.
• ResourceCost, CostResourceType, CostType models; AdminDb CRUD + admin_list_resource_costs_active_between; mysql + mock impls.

Admin API

Dedicated cost endpoints (resource_cost permission):

• GET/POST /api/admin/v1/resource_costs
• GET/PATCH/DELETE /api/admin/v1/resource_costs/{id} (PATCH-clear semantics for interval/billing fields)

Profit/Loss report

GET /api/admin/v1/reports/profit-loss (modelled on the sales time-series report, analytics::view):

• Per-period (month/year) rows, reported in a single target currency (currency param or the company's base currency).
• Revenue uses each payment's stored historical rate to value it in its company base currency (never live rates); a base→report-currency step only applies when aggregating companies with differing base currencies.
• Costs (no stored rate) are converted with current exchange rates; recurring costs normalized per active calendar month, ip_range per-IP costs scaled by the range's current assigned-IP count, one-time costs booked in their billing_start period.
• Optional company_id and region_id filters.

Deferred (follow-up)

• Historical (per-month) IP assignment counts — currently uses the range's current assigned-IP count.
• Costs are global, not company-scoped (so company_id filters revenue only).

Testing

• cargo build --workspace clean; cargo test -p lnvps_db model roundtrip tests pass (incl. new ResourceCost/CostType enum roundtrips).

Docs

• ADMIN_API_ENDPOINTS.md and API_CHANGELOG.md updated.