Replace ab with locust for realistic load testing

## Problem

Current `stability_test.py` uses Apache Bench which sends **identical payloads** repeatedly:
- Database caching heavily skews results
- Doesn't reveal performance differences based on UTXO count, transaction history, or block size
- Can't vary data per request
- Unrealistic compared to production traffic patterns

## Proposed Solution

**Tool**: Locust (Python-based load testing framework)

**Key Benefits**:
- Read CSV with multiple rows, cycle through different data per request
- Built-in p95/p99 percentile metrics
- Real-time web UI for monitoring
- Can weight endpoints differently (e.g., account/balance gets 10x more traffic than construction/metadata)
- CI/CD friendly headless mode

## Data Strategy

Create varied CSV files with diverse data profiles:

**Address Diversity:**
- Light (1-10 UTXOs), Medium (100-1K UTXOs), Heavy (10K+ UTXOs)
- New addresses (few txs) vs old addresses (thousands of txs)

**Block Diversity:**
- Light blocks (1-5 txs) vs heavy blocks (100+ txs)
- Recent blocks vs historical blocks

**Transaction Diversity:**
- Small (<500 bytes) vs large (>10KB)
- With/without metadata, with/without native assets