feat(analytics): add asset flow visualization engine

[yusuftomilola]

Summary

Implements the Asset Flow Visualization Engine under �pps/backend/src/modules/analytics/flows/.

Changes

• interfaces/asset-flow.interface.ts — defines AssetTransfer, FlowNode, FlowEdge, AssetFlowGraph, and AssetFlowQuery
• �sset-flow.service.ts — AssetFlowService with three methods:
  • generateGraph(transfers) — builds a directed graph of nodes (unique addresses) and edges (address→address per asset) with aggregated amounts and tx counts
  • queryFlow(transfers, query) — filters by address, chain, asset, and date range before generating the graph

  • raceHistory(transfers, address) — returns all transfers for an address sorted chronologically
    

• �sset-flow.module.ts — NestJS module exporting AssetFlowService
• �sset-flow.service.spec.ts — unit tests covering graph generation, node/edge aggregation, asset filtering, date range filtering, historical tracing, and edge cases

What was tested

All unit tests pass locally. Follows existing module conventions (reflect-metadata, NestJS Testing, Prettier config).

Closes #142

[mijinummi]

Hello @yusuftomilola , thank you for your contribution. Please kindly fix linting that are failing please