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/crud/syndarix/__init__.py

@@ -0,0 +1,20 @@
+# app/crud/syndarix/__init__.py
+"""
+Syndarix CRUD operations.
+
+This package contains CRUD operations for all Syndarix domain entities.
+"""
+
+from .agent_instance import agent_instance
+from .agent_type import agent_type
+from .issue import issue
+from .project import project
+from .sprint import sprint
+
+__all__ = [
+    "agent_instance",
+    "agent_type",
+    "issue",
+    "project",
+    "sprint",
+]