⚡ optimize participant update with batch chunked multi-row insert

[iwismer]

💡 What: Replaced loop-over-execute N+1 queries in replace_participants and replace_rd_data in services/receiver/src/db.rs with chunked batch multi-row INSERT OR REPLACE INTO participants operations (chunks of 50 items = 300 bind parameters) using cached prepared statements.

🎯 Why: Executing individual insert statements per participant in a loop incurs significant statement parsing and binding overhead for large race participant datasets (e.g., 10,000+ items).

📊 Measured Improvement:

• In cargo test --release --package receiver --lib -- db::tests::benchmark_bulk_operations:
  • Baseline replace_participants (10,000 records): ~10.25 ms
  • Optimized replace_participants (10,000 records): ~8.36 ms (~18.4% speedup)
  • replace_rd_data (10,000 participants, 10,000 chips, 10,000 divisions): ~31.10 ms baseline -> ~29.41 ms optimized (~5.4% overall speedup)
• All 381+ receiver unit and integration tests pass cleanly.

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PR created automatically by Jules for task 1468292792747040316 started by @iwismer

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