feat: Add Signet API endpoints for orders and fills

[init4samwise]

Summary

Implements REST API endpoints for querying Signet orders and fills by block number or transaction hash.

Endpoints Added

Block-based endpoints

• GET /api/v2/signet/blocks/:block_number/orders - Returns all orders initiated in a specific block
• GET /api/v2/signet/blocks/:block_number/fills - Returns all fills executed in a specific block
• GET /api/v2/signet/blocks/:block_number/activity - Combined view of orders and fills

Transaction-based endpoints

• GET /api/v2/signet/transactions/:tx_hash/orders - Orders initiated by a specific transaction
• GET /api/v2/signet/transactions/:tx_hash/fills - Fills executed by a specific transaction
• GET /api/v2/signet/transactions/:tx_hash/activity - Combined view for a transaction

Features

• ✅ Pagination support for all list endpoints
• ✅ JSON response format following Blockscout conventions
• ✅ Order status calculation (pending/expired based on deadline)
• ✅ Combined activity view for orders and fills together

Response Formats

Orders endpoint

{
  "items": [
    {
      "order_hash": "0x...:3",
      "outputs_witness_hash": "0x...",
      "deadline": 1707000000,
      "transaction_hash": "0x...",
      "log_index": 3,
      "block_number": 12345,
      "inputs": [{ "token": "0x...", "amount": "1000000" }],
      "outputs": [{ "token": "0x...", "amount": "500000", "recipient": "0x...", "chain_id": 1 }],
      "status": "pending",
      "sweep": null
    }
  ],
  "next_page_params": null
}

Fills endpoint

{
  "items": [
    {
      "outputs_witness_hash": "0x...",
      "transaction_hash": "0x...",
      "log_index": 5,
      "block_number": 12345,
      "chain_type": "rollup",
      "outputs": [{ "token": "0x...", "amount": "500000", "recipient": "0x...", "chain_id": 1 }]
    }
  ],
  "next_page_params": null
}

Files Changed

New files

• apps/explorer/lib/explorer/chain/signet.ex - Query functions for orders/fills
• apps/block_scout_web/lib/block_scout_web/controllers/api/v2/signet/signet_controller.ex - API controller
• apps/block_scout_web/lib/block_scout_web/views/api/v2/signet_view.ex - JSON view

Modified files

• apps/block_scout_web/lib/block_scout_web/routers/api_router.ex - Added Signet routes
• apps/explorer/lib/explorer/chain.ex - Added pagination helpers for Signet

Testing

Manual testing recommended with existing Signet data. All endpoints follow established Blockscout patterns.

---

Closes ENG-1896

[rymcol]

@init4samwise fix the failing checks here please