- Add FluentValidation validators for CreateFilamentRequest and UpdateFilamentRequest
with comprehensive validation rules (required fields, string lengths, hex color format,
weight constraints including WeightRemainingGrams <= WeightTotalGrams, purchase price range)
- Add FluentValidationFilter action filter that auto-runs FluentValidation validators
for all API controller actions before execution, returning 400 with structured error details
- Register FluentValidationFilter in DI and add it to MVC controller filters in Program.cs
- PUT endpoint was already implemented in FilamentsController with proper validation,
404 handling, FK existence checks, serial uniqueness check, and weight constraint check
- This change ensures FluentValidation rules are enforced consistently via the pipeline
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