feat: support int and numeric string timestamps in date filter

[osteele]

Summary

• Add Unix timestamp support (int, uint, float types) in Convert() for time.Time targets
• Parse numeric strings as Unix timestamps in ParseDate()
• Matches Shopify Liquid behavior where integer values are treated as Unix timestamps

Fixes #110

Test plan

• Unit tests for int, int64, float64 -> time.Time conversion
• Unit tests for numeric string parsing in ParseDate
• All existing tests pass