test(P14.1): add comprehensive backend API test suite

Add 134 tests as independent test project:
- test_auth.py (5): Login, JWT, protected endpoints
- test_users.py (8): User CRUD, permissions
- test_projects.py (8): Project CRUD, ownership
- test_milestones.py (7): Milestone CRUD, filtering
- test_tasks.py (8): Task CRUD, filtering
- test_comments.py (5): Comment CRUD, permissions
- test_roles.py (9): Role/permission management
- test_milestone_actions.py (17): Milestone state machine
- test_task_transitions.py (34): Task state machine
- test_propose.py (19): Propose CRUD, lifecycle
- test_misc.py (14): Notifications, activity, API keys, dashboard

Setup:
- conftest.py: SQLite in-memory DB, fixtures
- requirements.txt: Dependencies
- pyproject.toml: Pytest config
- README.md: Documentation

requirements.txt

@@ -0,0 +1,22 @@
+# HarborForge.Backend.Test dependencies
+# Tests the HarborForge.Backend as a separate project
+
+# Backend dependencies (must match Backend/requirements.txt)
+fastapi==0.109.0
+uvicorn[standard]==0.27.0
+sqlalchemy==2.0.25
+pymysql==1.1.0
+pydantic==2.5.3
+pydantic-settings==2.1.0
+python-jose[cryptography]==3.3.0
+passlib[bcrypt]==1.7.4
+bcrypt==4.0.1
+python-multipart==0.0.6
+alembic==1.13.1
+python-dotenv==1.0.0
+httpx==0.27.0
+requests==2.31.0
+
+# Test dependencies
+pytest==8.0.0
+pytest-asyncio==0.23.0