Refactor favorite status to type-safe enum and fix DateTime comparison

[Copilot]

Addresses PR review feedback on the My Festival feature by replacing string literals with type-safe enums and fixing timestamp comparison issues.

Type Safety

• Introduced FavoriteStatus enum (wantToTry, tasted) with fromString() factory for JSON deserialization
• Replaced all string literal status values throughout models, services, repositories, and tests
• Updated getFavoriteStatus() to return .value from enum

DateTime Comparison Fix

• Changed deleteTry to compare timestamps via millisecondsSinceEpoch instead of direct equality
• Prevents false matches when DateTime objects are deserialized from JSON with potentially different microsecond precision

Test Coverage

• Added test verifying deleteTry works correctly after JSON round-trip serialization
• Added delays between rapid timestamp creation to ensure distinct millisecond values
• Fixed test assertion type from Map<String, dynamic> to Map<String, FavoriteItem>

Documentation

• Documented FavoriteItem equality operator's ID-only comparison rationale (enables use in Sets/Maps where uniqueness is by drink, not state)
• Added comprehensive Optional<T> class documentation with usage examples for nullable copyWith parameters

// Before: error-prone string literals
status: 'want_to_try'

// After: type-safe enum
status: FavoriteStatus.wantToTry

// Before: fragile DateTime comparison
updatedTries = tries.where((t) => t != timestamp)

// After: reliable millisecond comparison  
updatedTries = tries.where((t) => t.millisecondsSinceEpoch != timestamp.millisecondsSinceEpoch)

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