feat(backend): Add Syndarix domain models with CRUD operations

- Add Project model with slug, description, autonomy level, and settings
- Add AgentType model for agent templates with model config and failover
- Add AgentInstance model for running agents with status and memory
- Add Issue model with external tracker sync (Gitea/GitHub/GitLab)
- Add Sprint model with velocity tracking and lifecycle management
- Add comprehensive Pydantic schemas with validation
- Add full CRUD operations for all models with filtering/sorting
- Add 280+ tests for models, schemas, and CRUD operations

Implements #23, #24, #25, #26, #27

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/tests/schemas/syndarix/conftest.py

@@ -0,0 +1,68 @@
+# tests/schemas/syndarix/conftest.py
+"""
+Shared fixtures for Syndarix schema tests.
+"""
+
+import uuid
+from datetime import date, timedelta
+
+import pytest
+
+
+@pytest.fixture
+def valid_uuid():
+    """Return a valid UUID for testing."""
+    return uuid.uuid4()
+
+
+@pytest.fixture
+def valid_project_data():
+    """Return valid project data for schema testing."""
+    return {
+        "name": "Test Project",
+        "slug": "test-project",
+        "description": "A test project",
+    }
+
+
+@pytest.fixture
+def valid_agent_type_data():
+    """Return valid agent type data for schema testing."""
+    return {
+        "name": "Backend Engineer",
+        "slug": "backend-engineer",
+        "personality_prompt": "You are an expert backend engineer.",
+        "primary_model": "claude-opus-4-5-20251101",
+    }
+
+
+@pytest.fixture
+def valid_sprint_data(valid_uuid):
+    """Return valid sprint data for schema testing."""
+    today = date.today()
+    return {
+        "project_id": valid_uuid,
+        "name": "Sprint 1",
+        "number": 1,
+        "start_date": today,
+        "end_date": today + timedelta(days=14),
+    }
+
+
+@pytest.fixture
+def valid_issue_data(valid_uuid):
+    """Return valid issue data for schema testing."""
+    return {
+        "project_id": valid_uuid,
+        "title": "Test Issue",
+        "body": "Issue description",
+    }
+
+
+@pytest.fixture
+def valid_agent_instance_data(valid_uuid):
+    """Return valid agent instance data for schema testing."""
+    return {
+        "agent_type_id": valid_uuid,
+        "project_id": valid_uuid,
+    }