[i18n] Extract hardcoded error strings to resource files

[iammukeshm]

Problem

Error messages are hardcoded throughout the codebase.

Examples

throw new NotFoundException("$"Group with ID '{command.Id}' not found.");
throw new CustomException("Toggle status failed", errors);
await context.Response.WriteAsync("Endpoint not found.");

Files affected

• All handlers with validation
• Exception messages
• Response messages

Suggested approach

1. Create Resources/ErrorMessages.resx
2. Use IStringLocalizer<T> for messages
3. Create constants for error codes

Impact

• Cannot localize application
• Inconsistent error messages

[iammukeshm]

Clearing the pre-v10 backlog as part of the v10 (.NET 10) reset. This is still relevant — re-filed cleanly for v10 as #1251. Closing this one.