sorted the /assets/popular endpoint

[ALLEN-AYODEJI]

src/routes/assets/popular.ts (new) — GET /assets/popular?window=24h&by=transfers

• Validates window (1h/24h/7d) and by (transfers/volume)
• Defaults: window=24h, by=transfers, limit=20, offset=0
  src/db.ts (modified)
• Exported toDisplayAmount
• Added queryPopularAssets — raw SQL aggregation: GROUP BY contractId, COUNT(*) for transfer count, SUM(CAST(amount AS NUMERIC)) for volume, ordered by the by param
  src/api.ts (modified) — mounted router at /assets
  Response shape:
  {
  "window": "24h",
  "by": "transfers",
  "limit": 20,
  "offset": 0,
  "total": 50,
  "assets": [
  { "contractId": "C...", "transferCount": 1234, "volume": "5000000000000000", "displayVolume": "500.0000000" }
  ]
  }

Closes #109

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