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/syndarix/__init__.py

@@ -0,0 +1,41 @@
+# app/models/syndarix/__init__.py
+"""
+Syndarix domain models.
+
+This package contains all the core entities for the Syndarix AI consulting platform:
+- Project: Client engagements with autonomy settings
+- AgentType: Templates for AI agent capabilities
+- AgentInstance: Spawned agents working on projects
+- Issue: Units of work with external tracker sync
+- Sprint: Time-boxed iterations for organizing work
+"""
+
+from .agent_instance import AgentInstance
+from .agent_type import AgentType
+from .enums import (
+    AgentStatus,
+    AutonomyLevel,
+    IssuePriority,
+    IssueStatus,
+    ProjectStatus,
+    SprintStatus,
+    SyncStatus,
+)
+from .issue import Issue
+from .project import Project
+from .sprint import Sprint
+
+__all__ = [
+    "AgentInstance",
+    "AgentStatus",
+    "AgentType",
+    "AutonomyLevel",
+    "Issue",
+    "IssuePriority",
+    "IssueStatus",
+    "Project",
+    "ProjectStatus",
+    "Sprint",
+    "SprintStatus",
+    "SyncStatus",
+]