Add Gear entity to track brewing equipment (grinders and brewers) with complete domain layer implementation.
Database changes:
- migrations/0006_add_gear.sql: Create gear table with category CHECK constraint and indexes
- migrations/0007_update_timeline_for_gear.sql: Document 'gear' as valid timeline entity type
Domain layer:
- Add GearId typed ID wrapper
- Create domain/gear.rs with:
- GearCategory enum (Grinder/Brewer) with string conversion methods
- Gear entity with make, model, notes fields
- NewGear and UpdateGear DTOs
- GearFilter for category-based filtering
- GearSortKey with Make (default), Model, Category, CreatedAt options
- Add GearRepository trait to domain/repositories.rs with standard CRUD operations
- Register gear module in domain/mod.rs
This follows the same architectural pattern as the Bag entity.
