feat(model-prices): support non-token billing metadata for image and video models

[mazongYY]

Summary

Adds optional billing metadata (billingUnit / billingRate) to stored model prices so media models can record official non-token pricing while token cost stays at 0.

Targeted xAI media models:

• grok-imagine-image: $0.02/image
• grok-imagine-video: $0.05/s (480p), $0.07/s (720p)

Changes

• model.ModelPrice and repository SQL persist billing_unit / billing_rate, including sync upsert and manual replace paths.
• SQLite table creation and column-preservation migration include the two nullable columns; existing installs migrate in place.
• The web editor exposes Billing unit and Billing rate inputs; price rows display the billing rate when present.
• i18n keys added for en, zh-CN, zh-TW, and ru.
• Tests cover repository round-trip, legacy migration preserving zero-base prices with no billing metadata, and draft round-trip / blank-metadata omission.

Validation

• go test ./internal/repository/sqlite/...
• go test ./internal/repository/modelprice/...
• go test ./internal/service/modelprice/...
• go test ./internal/model/...
• go build ./...
• npm run type-check
• npm run lint
• npx vitest run src/features/monitoring/model/modelPricesPageModel.test.ts
• git diff --check

Fixes #519

[mazongYY]

Hi maintainers 👋 — this PR has been rebased onto the latest dev (cdc97ce), so it should apply cleanly.

Local validation on the rebased head (3c26e6a):

• go test ./internal/repository/sqlite/... ✅
• go test ./internal/repository/modelprice/... ✅ (incl. new billing-metadata round-trip)
• go test ./internal/service/modelprice/... ./internal/model/... ✅
• go build ./... ✅
• npm run type-check ✅ / npm run lint (0 errors) ✅
• vitest run src/features/monitoring/model/modelPricesPageModel.test.ts (12/12) ✅

Note: the PR Check workflow shows action_required — as this is a fork PR, the checks need a maintainer approval to run. Could you approve the workflow run and, if everything looks good, merge this? It closes #519. Thanks!