Fix Unicode case-insensitive search for keyword queries

[sadorlovsky]

Problem

The original implementation used the LIKE operator for case-insensitive search which only supports case-insensitive matching for ASCII characters (A-Z), not Unicode. SQLite's LOWER() function also lacks Unicode support, so it wouldn't solve the problem. This caused searches to be case-sensitive for non-English content.

Example:

• Searching for "анна" would NOT find "Анна"
• Searching for "chloé" would NOT find "CHLOÉ"
• Searching for "έλενα" would NOT find "Έλενα"

Solution

Use custom SQLite function lower_unicode() that uses Go's strings.ToLower() for proper Unicode case folding. All keyword search queries now use this function to ensure truly case-insensitive matching across all character sets.

[WithoutPants]

I think we might want to consider adding/changing name indexes to accommodate this.