feat(assets): CSV import of the asset inventory #861

Problem

A team moving to OpenRisk already has its asset inventory in a spreadsheet. Today the only way in is the form, one asset at a time. #755 lets the risk import link risks to assets by name, which makes the gap more visible: you have to type every asset by hand before your risk file can reference it. The original stub (internal/service/migration_service.go, TODO: Implement actual asset import logic) was never reachable, and the #755 audit split asset import out.

Acceptance criteria

1. POST /assets/import (multipart file, CSV) is mounted behind asset:create and capAssets, and is scoped to the session's tenant_id. Tenant and author never come from the request.
2. Columns: name (required, ≤ 255), type, criticality (LOW/MEDIUM/HIGH/CRITICAL, case-insensitive, plus the French FAIBLE/MOYENNE/ÉLEVÉE/CRITIQUE; default MEDIUM), owner. Unknown columns are refused and named. Typed category/attributes are out of scope.
3. All-or-nothing. Every row is validated first. One invalid row writes nothing, and the response lists every error with line, column, code, params and message, in the same contract as fix(risks): CSV import of risks and assets fails silently #755. A valid file is written in one transaction through CreateAssetUseCase.
4. A name repeated in the file, or matching an existing asset of the tenant (case-insensitive), is refused with its line. Re-importing the same file never duplicates the inventory.
5. The plan limit is checked against the whole file (402 limit_reached). Nothing is written.
6. Same file handling as fix(risks): CSV import of risks and assets fails silently #755: UTF-8 BOM, ; delimiter, 1000 rows / 2 MB max.
7. UI: an import page reachable from the inventory, with a downloadable template, FR/EN rendering of every error code, the three UI states, and no success toast unless created > 0.
8. Tests: Success, Validation (bad row → zero rows), Unauthorized (no asset:create → 403), NotFound (no file), cross-tenant (rows land only in the caller's tenant; another tenant's names do not count as duplicates).

Definition of Done

Criteria 1–8 verified by tests and by a live pass on Postgres. PR open with Closes #<n>, stacked on #859.