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/app/models/__init__.py

@@ -23,6 +23,15 @@ from .user import User
 from .user_organization import OrganizationRole, UserOrganization
 from .user_session import UserSession
 
+# Syndarix domain models
+from .syndarix import (
+    AgentInstance,
+    AgentType,
+    Issue,
+    Project,
+    Sprint,
+)
+
 __all__ = [
     "Base",
     "OAuthAccount",
@@ -38,4 +47,10 @@ __all__ = [
     "User",
     "UserOrganization",
     "UserSession",
+    # Syndarix models
+    "AgentInstance",
+    "AgentType",
+    "Issue",
+    "Project",
+    "Sprint",
 ]