feat: Implement domain logic separation with clean architecture

- Created complete domain layer with pure business logic
- Implemented Issue domain models with 48 passing tests
- Implemented Project domain models with 31 passing tests
- Added domain services for complex business operations
- Established clean separation between domain, application, and infrastructure
- All 250 tests passing with no breaking changes

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

domain/projects/models.py

@@ -0,0 +1,162 @@
+"""
+Project domain models.
+
+Contains core business entities and value objects for project management.
+"""
+
+from dataclasses import dataclass
+from typing import List, Optional, Dict, Any
+from datetime import datetime
+from enum import Enum
+
+from .exceptions import MilestoneError
+
+
+class ProjectState(Enum):
+    """Project state enumeration."""
+    ACTIVE = "active"
+    ARCHIVED = "archived"
+    PLANNING = "planning"
+
+
+@dataclass
+class Milestone:
+    """Milestone entity."""
+    id: int
+    title: str
+    description: Optional[str]
+    due_date: Optional[datetime]
+    state: str
+    open_issues: int
+    closed_issues: int
+
+    @property
+    def completion_percentage(self) -> float:
+        """Calculate milestone completion percentage."""
+        total_issues = self.open_issues + self.closed_issues
+        if total_issues == 0:
+            return 0.0
+        return (self.closed_issues / total_issues) * 100
+
+    @property
+    def total_issues(self) -> int:
+        """Get total number of issues in milestone."""
+        return self.open_issues + self.closed_issues
+
+    def is_overdue(self) -> bool:
+        """Check if milestone is overdue."""
+        if not self.due_date or self.state == "closed":
+            return False
+        return datetime.utcnow() > self.due_date
+
+    def is_completed(self) -> bool:
+        """Check if milestone is completed."""
+        return self.state == "closed" or (self.total_issues > 0 and self.completion_percentage >= 100.0)
+
+    def add_issue(self) -> None:
+        """Add an open issue to the milestone."""
+        self.open_issues += 1
+
+    def close_issue(self) -> None:
+        """Close an issue in the milestone."""
+        if self.open_issues <= 0:
+            raise MilestoneError(
+                f"Cannot close issue in milestone '{self.title}': no open issues",
+                milestone_id=self.id
+            )
+        self.open_issues -= 1
+        self.closed_issues += 1
+
+    def reopen_issue(self) -> None:
+        """Reopen an issue in the milestone."""
+        if self.closed_issues <= 0:
+            raise MilestoneError(
+                f"Cannot reopen issue in milestone '{self.title}': no closed issues",
+                milestone_id=self.id
+            )
+        self.closed_issues -= 1
+        self.open_issues += 1
+
+
+@dataclass
+class Project:
+    """Project aggregate root."""
+    name: str
+    description: str
+    state: ProjectState
+    milestones: List[Milestone]
+    kanban_columns: List[str]
+    created_at: datetime
+    updated_at: datetime
+    archived_at: Optional[datetime] = None
+
+    def get_active_milestones(self) -> List[Milestone]:
+        """Get milestones that are currently active."""
+        return [milestone for milestone in self.milestones if milestone.state == "open"]
+
+    def get_completed_milestones(self) -> List[Milestone]:
+        """Get milestones that are completed."""
+        return [milestone for milestone in self.milestones if milestone.is_completed()]
+
+    def get_overdue_milestones(self) -> List[Milestone]:
+        """Get milestones that are overdue."""
+        return [milestone for milestone in self.milestones if milestone.is_overdue()]
+
+    def calculate_overall_progress(self) -> float:
+        """Calculate overall project progress based on milestones."""
+        if not self.milestones:
+            return 0.0
+
+        total_completion = sum(milestone.completion_percentage for milestone in self.milestones)
+        return total_completion / len(self.milestones)
+
+    def get_total_issues(self) -> int:
+        """Get total number of issues across all milestones."""
+        return sum(milestone.total_issues for milestone in self.milestones)
+
+    def get_total_open_issues(self) -> int:
+        """Get total number of open issues across all milestones."""
+        return sum(milestone.open_issues for milestone in self.milestones)
+
+    def get_total_closed_issues(self) -> int:
+        """Get total number of closed issues across all milestones."""
+        return sum(milestone.closed_issues for milestone in self.milestones)
+
+    def archive(self) -> None:
+        """Archive the project."""
+        if self.state == ProjectState.ARCHIVED:
+            return  # Already archived
+
+        self.state = ProjectState.ARCHIVED
+        self.archived_at = datetime.utcnow()
+
+    def activate(self) -> None:
+        """Activate the project."""
+        if self.state == ProjectState.ACTIVE:
+            return  # Already active
+
+        self.state = ProjectState.ACTIVE
+        self.archived_at = None
+
+    def add_milestone(self, milestone: Milestone) -> None:
+        """Add a milestone to the project."""
+        # Check for duplicate milestone IDs
+        if any(m.id == milestone.id for m in self.milestones):
+            raise ValueError(f"Milestone with ID {milestone.id} already exists")
+
+        self.milestones.append(milestone)
+
+    def remove_milestone(self, milestone_id: int) -> None:
+        """Remove a milestone from the project."""
+        original_count = len(self.milestones)
+        self.milestones = [m for m in self.milestones if m.id != milestone_id]
+
+        if len(self.milestones) == original_count:
+            raise ValueError(f"Milestone with ID {milestone_id} not found")
+
+    def get_milestone(self, milestone_id: int) -> Optional[Milestone]:
+        """Get a milestone by ID."""
+        for milestone in self.milestones:
+            if milestone.id == milestone_id:
+                return milestone
+        return None