feat: add ClassifiedAd domain entity

.agent/current-plan.md

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+# Plan: ClassifiedAd Domain Entity
+
+## Status: COMPLETED
+
+## Objective
+Create the first Domain Entity representing a ClassifiedAd with the specified attributes.
+
+## Decisions Made
+- **Location**: `domain/marketplace/classified_ad.py`
+- **Class style**: `@dataclass(frozen=True)` for immutability
+- **Validation**: No validation, just hold data
+- **Tests**: Yes, create unit tests
+
+## Entity Structure
+```python
+@dataclass(frozen=True)
+class ClassifiedAd:
+    id: UUID
+    _ownerId: UUID
+    title: str
+    text: str
+    _price: Decimal
+```
+
+## Files Created
+- `domain/__init__.py`
+- `domain/marketplace/__init__.py`
+- `domain/marketplace/classified_ad.py`
+- `tests/test_classified_ad.py`
+- `pyproject.toml`
+
+## Test Results
+- `test_classified_ad_creation` - PASSED
+- `test_classified_ad_is_immutable` - PASSED
+- `test_classified_ad_equality` - PASSED