Add index on mktOrders for market speedup

[kenneaal]

This adds an SQL migration that adds an index on solarSystemID to mktOrders. This significantly speeds up market browsing.

[sourcery-ai]

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[sourcery-ai]

Hey @kenneaal - I've reviewed your changes and they look great!

Here's what I looked at during the review

• 🟢 General issues: all looks good
• 🟢 Security: all looks good
• 🟢 Testing: all looks good
• 🟢 Complexity: all looks good
• 🟢 Documentation: all looks good

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[jdhirst]

@kenneaal thanks for the contribution, looks good to me!