Batch database upserts to eliminate N×M sequential queries

[Copilot]

Addresses performance feedback from #45 where nested loops with sequential await create N×M database round-trips during international price updates.

Changes:

• Collect all upsert operations in an array during loop iteration
• Execute batched operations with Promise.all after loop completion
• Reduces database queries from N×M sequential to 2 initial + 1 parallel batch

Before:

for (const { currency, rate } of rates) {
  for (const offer of templateOffers) {
    await prisma.coinOffers.upsert({ ... }); // N×M sequential queries
  }
}

After:

const upsertOperations = [];
for (const { currency, rate } of rates) {
  for (const offer of templateOffers) {
    upsertOperations.push(prisma.coinOffers.upsert({ ... }));
  }
}
await Promise.all(upsertOperations); // Single parallel batch

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