feat: WebSearch tri-state, account stats pricing fix, quota cache fix, usage tooltip

WebSearch tri-state switch:
- Account-level web_search_emulation changed from bool to tri-state
  string: "default" (follow channel) / "enabled" / "disabled"
- shouldEmulateWebSearch checks channel config when account is "default"
- SQL migration converts old bool values
- Frontend select replaces toggle in Edit/CreateAccountModal

Account stats pricing:
- resolveAccountStatsCost uses upstream model (post-mapping) for matching
- Priority: custom rules → model pricing file (when toggle on) → default
- Custom rules always configurable, independent of toggle
- Account ID field changed to searchable selector filtered by platform
- Description updated to reflect new behavior

Quota notification cache fix:
- CheckAccountQuotaAfterIncrement fetches real-time account from DB
- Reconstructs pre-increment usage for accurate threshold crossing detection
- New AccountQuotaReader interface (minimal: GetByID only)

Usage tooltip:
- Per-request/image billing shows per-request price instead of $0 token price
- Token billing continues to show input/output price per million tokens

backend/internal/service/openai_gateway_service.go

@@ -4580,7 +4580,7 @@ func (s *OpenAIGatewayService) RecordUsage(ctx context.Context, input *OpenAIRec
 			statsModel = result.Model
 		}
 		usageLog.AccountStatsCost = resolveAccountStatsCost(
-			ctx, s.channelService,
+			ctx, s.channelService, s.billingService,
 			account.ID, *apiKey.GroupID, statsModel,
 			tokens, 1,
 		)