Potential fix for code scanning alert no. 5: Size computation for allocation may overflow

[bsdrop]

Potential fix for https://github.com/bsdrop/lafdb/security/code-scanning/5

To fix this safely without changing functionality, compute the combined capacity once with an overflow guard before using it in make. In Go, the standard pattern is:

• check if a > math.MaxInt-b before a+b
• if overflow would occur, return an error instead of allocating

Best concrete fix here:

1. In internal/scraper/modified.go, import math.
2. In buildModifiedEvents, before allocations on lines currently using len(oldMap)+len(newMap), add:
   • oldLen := len(oldMap)
   • newLen := len(newMap)
   • overflow guard against math.MaxInt
   • combinedLen := oldLen + newLen
3. Use combinedLen for both ids and seen capacities.
4. Return an error (using existing fmt.Errorf) if overflow is detected.

This addresses all three alert variants because they all flow into the same vulnerable sink in buildModifiedEvents.

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