[FEATURE] Transaction History API — Query blockchain transactions with filtering

[gelluisaac]

Description

Build the REST API for querying blockchain transaction history with rich filtering.
This feeds the TransactionHistoryPage component in the web frontend.

Endpoints

• GET /api/v1/transactions — List transactions with filters
  • Query params: source_account, destination_account, asset_code,
    start_date, end_date, min_amount, max_amount, operation_type,
    successful, page, page_size
• GET /api/v1/transactions/{hash} — Get single transaction by hash
• GET /api/v1/transactions/stats — Aggregated stats (total volume, count by asset, etc.)

Implementation Details

• Use SQLAlchemy filtering with parameterized queries
• Indexes on: (source_account, timestamp), (destination_account, timestamp), hash
• Response schema matches BlockchainTransaction / TransactionHistoryResponse in web/src/lib/types.ts

Acceptance Criteria

• All filter parameters work correctly
• Compound filters (e.g., source + date range) are efficient
• Missing optional filters default to no filtering
• Transaction stats return correctly aggregated data

Labels

enhancement, api