fix(overrides): address gaps in attribute override system

[Starosdev]

Summary

Several verified gaps were found in the attribute override system during a code review against upstream discussion AnalogJ#922.

Why

These issues affect correctness (silent duplicate overrides), visibility (config file overrides invisible in UI), safety (missing threshold validation), and confidence (no handler-level tests).

Acceptance Criteria

• Add unique index on (protocol, attribute_id, wwn) in the attribute_overrides migration to prevent silent duplicates
• Config file overrides are returned by GET /api/settings/overrides so they appear in the settings UI (read-only, source: config)
• Handler validates that warn_above < fail_above, values are non-negative, and at least one threshold is set when action == ""
• Add handler-level tests for GET/POST/DELETE /api/settings/overrides
• UI form default action changed from ignore to "" (Custom Threshold) so threshold fields are visible by default; form resets to same
• Fix misleading comment in applier.go that says thresholds "can be combined with force_status" — they are mutually exclusive in practice
• Add basic WWN format validation in handler

Additional Context

• Migration file: webapp/backend/pkg/database/migrations/m20260122000000/attribute_override.go
• Handler: webapp/backend/pkg/web/handler/attribute_overrides.go
• Applier: webapp/backend/pkg/overrides/applier.go
• DB repo: webapp/backend/pkg/database/scrutiny_repository_overrides.go
• UI: webapp/frontend/src/app/layout/common/dashboard-settings/dashboard-settings.component.ts