fix: Eliminate all 111 test warnings by fixing root causes

- Replace deprecated datetime.utcnow() with datetime.now(timezone.utc)
  across all domain models, services, infrastructure, and test files
- Add missing timezone imports to all affected files
- Fix pytest.ini configuration format from [tool:pytest] to [pytest]
- Remove warning suppressions to expose actual issues
- Ensure proper pytest marker registration for smoke tests

Results:
- 305 passed, 2 skipped, 0 warnings (down from 111 warnings)
- All functionality preserved with modern datetime API usage
- Improved code quality by addressing root causes vs suppression

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

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

domain/issues/models.py

@@ -6,7 +6,7 @@ Contains core business entities and value objects for issue management.
 
 from dataclasses import dataclass
 from typing import List, Optional
-from datetime import datetime
+from datetime import datetime, timezone
 from enum import Enum
 
 from .exceptions import IssueStateError
@@ -88,7 +88,7 @@ class Issue:
             )
 
         self.state = IssueState.CLOSED
-        self.closed_at = datetime.utcnow()
+        self.closed_at = datetime.now(timezone.utc)
 
     def reopen(self) -> None:
         """Reopen the issue - domain business rule."""