✨ feat(models-sync): official upstream sync with conflict resolution UI, opt‑out flag, and backend resiliency

Backend
- Add endpoints:
  - GET /api/models/sync_upstream/preview — diff preview (filters out models with sync_official = 0)
  - POST /api/models/sync_upstream — apply sync (create missing; optionally overwrite selected fields)
- Respect opt‑out: skip models with sync_official = 0 in both preview and apply
- Return detailed stats: created_models, created_vendors, updated_models, skipped_models, plus created_list / updated_list
- Add model.Model.SyncOfficial (default 1); auto‑migrated by GORM
- Make HTTP fetching robust:
  - Shared http.Client (connection reuse) with 3x exponential backoff retry
  - 10MB response cap; keep existing IPv4‑first for *.github.io
- Vendor handling:
  - New ensureVendorID helper (cache lookup → DB lookup → create), reduces round‑trips
  - Transactional overwrite to avoid partial updates
- Small cleanups and clearer helpers (containsField, coalesce, chooseStatus)

Frontend
- ModelsActions: add “Sync official” button with Popover (p‑2) explaining community contribution; loading = syncing || previewing; preview → conflict modal → apply flow
- New UpstreamConflictModal:
  - Per‑field columns (description/icon/tags/vendor/name_rule/status) with column‑level checkbox to select all
  - Cell with Checkbox + Tag (“Click to view differences”) and Popover (p‑2) showing Local vs Official values
  - Auto‑hide columns with no conflicts; responsive width; use native Semi Modal footer
  - Full i18n coverage
- useModelsData: add syncing/previewing states; new methods previewUpstreamDiff, applyUpstreamOverwrite, syncUpstream; refresh vendors/models after apply
- EditModelModal: add “Participate in official sync” switch; persisted as sync_official
- ModelsColumnDefs: add “Participate in official sync” column

i18n
- Add missing English keys for the new UI and messages; fix quoting issues

Refs
- Upstream metadata: https://github.com/basellm/llm-metadata

model/model_meta.go

@@ -28,6 +28,7 @@ type Model struct {
 	VendorID    int            `json:"vendor_id,omitempty" gorm:"index"`
 	Endpoints   string         `json:"endpoints,omitempty" gorm:"type:text"`
 	Status      int            `json:"status" gorm:"default:1"`
+	SyncOfficial int           `json:"sync_official" gorm:"default:1"`
 	CreatedTime int64          `json:"created_time" gorm:"bigint"`
 	UpdatedTime int64          `json:"updated_time" gorm:"bigint"`
 	DeletedAt   gorm.DeletedAt `json:"-" gorm:"index;uniqueIndex:uk_model_name_delete_at,priority:2"`