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

backend/tests/models/syndarix/__init__.py

@@ -0,0 +1,2 @@
+# tests/models/syndarix/__init__.py
+"""Syndarix model unit tests."""