⚡ Batch insert participants, chips, and divisions in receiver database

[iwismer]

💡 What:
Optimized replace_participants, replace_bib_chips, replace_divisions, and replace_rd_data in services/receiver/src/db.rs to replace individual single-row insert loops with chunked multi-row INSERT OR REPLACE INTO ... VALUES (...), (...) queries (64 rows per chunk).

🎯 Why:
Previously, bulk imports iteratively executed single-row prepared statements inside loops, leading to significant N+1 statement execution overhead. Batching rows into multi-row insert statements reduces database statement execution calls by ~64x.

📊 Measured Improvement:
In release benchmarks with 10,000 records each:

• replace_divisions: reduced insertion time from ~17.0ms to ~4.96ms (~3.4x faster)
• replace_participants: reduced insertion time from ~27.3ms to ~12.5ms (~2.18x faster)
• replace_bib_chips: reduced insertion time from ~45.6ms to ~9.56ms (~4.77x faster)
• replace_rd_data: reduced combined replacement time from ~91.7ms to ~27.5ms (~3.33x faster)

---

PR created automatically by Jules for task 12042194851751262980 started by @iwismer

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.