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/agent_instance.py

@@ -0,0 +1,122 @@
+# app/schemas/syndarix/agent_instance.py
+"""
+Pydantic schemas for AgentInstance entity.
+"""
+
+from datetime import datetime
+from decimal import Decimal
+from typing import Any
+from uuid import UUID
+
+from pydantic import BaseModel, ConfigDict, Field
+
+from .enums import AgentStatus
+
+
+class AgentInstanceBase(BaseModel):
+    """Base agent instance schema with common fields."""
+
+    agent_type_id: UUID
+    project_id: UUID
+    status: AgentStatus = AgentStatus.IDLE
+    current_task: str | None = None
+    short_term_memory: dict[str, Any] = Field(default_factory=dict)
+    long_term_memory_ref: str | None = Field(None, max_length=500)
+    session_id: str | None = Field(None, max_length=255)
+
+
+class AgentInstanceCreate(BaseModel):
+    """Schema for creating a new agent instance."""
+
+    agent_type_id: UUID
+    project_id: UUID
+    status: AgentStatus = AgentStatus.IDLE
+    current_task: str | None = None
+    short_term_memory: dict[str, Any] = Field(default_factory=dict)
+    long_term_memory_ref: str | None = Field(None, max_length=500)
+    session_id: str | None = Field(None, max_length=255)
+
+
+class AgentInstanceUpdate(BaseModel):
+    """Schema for updating an agent instance."""
+
+    status: AgentStatus | None = None
+    current_task: str | None = None
+    short_term_memory: dict[str, Any] | None = None
+    long_term_memory_ref: str | None = None
+    session_id: str | None = None
+    last_activity_at: datetime | None = None
+    tasks_completed: int | None = Field(None, ge=0)
+    tokens_used: int | None = Field(None, ge=0)
+    cost_incurred: Decimal | None = Field(None, ge=0)
+
+
+class AgentInstanceTerminate(BaseModel):
+    """Schema for terminating an agent instance."""
+
+    reason: str | None = None
+
+
+class AgentInstanceInDB(AgentInstanceBase):
+    """Schema for agent instance in database."""
+
+    id: UUID
+    last_activity_at: datetime | None = None
+    terminated_at: datetime | None = None
+    tasks_completed: int = 0
+    tokens_used: int = 0
+    cost_incurred: Decimal = Decimal("0.0000")
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class AgentInstanceResponse(BaseModel):
+    """Schema for agent instance API responses."""
+
+    id: UUID
+    agent_type_id: UUID
+    project_id: UUID
+    status: AgentStatus
+    current_task: str | None = None
+    short_term_memory: dict[str, Any] = Field(default_factory=dict)
+    long_term_memory_ref: str | None = None
+    session_id: str | None = None
+    last_activity_at: datetime | None = None
+    terminated_at: datetime | None = None
+    tasks_completed: int = 0
+    tokens_used: int = 0
+    cost_incurred: Decimal = Decimal("0.0000")
+    created_at: datetime
+    updated_at: datetime
+
+    # Expanded fields from relationships
+    agent_type_name: str | None = None
+    agent_type_slug: str | None = None
+    project_name: str | None = None
+    project_slug: str | None = None
+    assigned_issues_count: int | None = 0
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class AgentInstanceListResponse(BaseModel):
+    """Schema for paginated agent instance list responses."""
+
+    agent_instances: list[AgentInstanceResponse]
+    total: int
+    page: int
+    page_size: int
+    pages: int
+
+
+class AgentInstanceMetrics(BaseModel):
+    """Schema for agent instance metrics summary."""
+
+    total_instances: int
+    active_instances: int
+    idle_instances: int
+    total_tasks_completed: int
+    total_tokens_used: int
+    total_cost_incurred: Decimal