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/schemas/syndarix/enums.py

@@ -0,0 +1,26 @@
+# app/schemas/syndarix/enums.py
+"""
+Re-export enums from models for use in schemas.
+
+This allows schemas to import enums without depending on SQLAlchemy models directly.
+"""
+
+from app.models.syndarix.enums import (
+    AgentStatus,
+    AutonomyLevel,
+    IssuePriority,
+    IssueStatus,
+    ProjectStatus,
+    SprintStatus,
+    SyncStatus,
+)
+
+__all__ = [
+    "AgentStatus",
+    "AutonomyLevel",
+    "IssuePriority",
+    "IssueStatus",
+    "ProjectStatus",
+    "SprintStatus",
+    "SyncStatus",
+]