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/agent_instance.py

@@ -0,0 +1,108 @@
+# app/models/syndarix/agent_instance.py
+"""
+AgentInstance model for Syndarix AI consulting platform.
+
+An AgentInstance is a spawned instance of an AgentType, assigned to a
+specific project to perform work.
+"""
+
+from sqlalchemy import (
+    BigInteger,
+    Column,
+    DateTime,
+    Enum,
+    ForeignKey,
+    Index,
+    Integer,
+    Numeric,
+    String,
+    Text,
+)
+from sqlalchemy.dialects.postgresql import (
+    JSONB,
+    UUID as PGUUID,
+)
+from sqlalchemy.orm import relationship
+
+from app.models.base import Base, TimestampMixin, UUIDMixin
+
+from .enums import AgentStatus
+
+
+class AgentInstance(Base, UUIDMixin, TimestampMixin):
+    """
+    AgentInstance model representing a spawned agent working on a project.
+
+    Tracks:
+    - Current status and task
+    - Memory (short-term in DB, long-term reference to vector store)
+    - Session information for MCP connections
+    - Usage metrics (tasks completed, tokens, cost)
+    """
+
+    __tablename__ = "agent_instances"
+
+    # Foreign keys
+    agent_type_id = Column(
+        PGUUID(as_uuid=True),
+        ForeignKey("agent_types.id", ondelete="RESTRICT"),
+        nullable=False,
+        index=True,
+    )
+
+    project_id = Column(
+        PGUUID(as_uuid=True),
+        ForeignKey("projects.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+
+    # Status tracking
+    status: Column[AgentStatus] = Column(
+        Enum(AgentStatus),
+        default=AgentStatus.IDLE,
+        nullable=False,
+        index=True,
+    )
+
+    # Current task description (brief summary of what agent is doing)
+    current_task = Column(Text, nullable=True)
+
+    # Short-term memory stored in database (conversation context, recent decisions)
+    short_term_memory = Column(JSONB, default=dict, nullable=False)
+
+    # Reference to long-term memory in vector store (e.g., "project-123/agent-456")
+    long_term_memory_ref = Column(String(500), nullable=True)
+
+    # Session ID for active MCP connections
+    session_id = Column(String(255), nullable=True, index=True)
+
+    # Activity tracking
+    last_activity_at = Column(DateTime(timezone=True), nullable=True, index=True)
+    terminated_at = Column(DateTime(timezone=True), nullable=True, index=True)
+
+    # Usage metrics
+    tasks_completed = Column(Integer, default=0, nullable=False)
+    tokens_used = Column(BigInteger, default=0, nullable=False)
+    cost_incurred = Column(Numeric(precision=10, scale=4), default=0, nullable=False)
+
+    # Relationships
+    agent_type = relationship("AgentType", back_populates="instances")
+    project = relationship("Project", back_populates="agent_instances")
+    assigned_issues = relationship(
+        "Issue",
+        back_populates="assigned_agent",
+        foreign_keys="Issue.assigned_agent_id",
+    )
+
+    __table_args__ = (
+        Index("ix_agent_instances_project_status", "project_id", "status"),
+        Index("ix_agent_instances_type_status", "agent_type_id", "status"),
+        Index("ix_agent_instances_project_type", "project_id", "agent_type_id"),
+    )
+
+    def __repr__(self) -> str:
+        return (
+            f"<AgentInstance {self.id} type={self.agent_type_id} "
+            f"project={self.project_id} status={self.status.value}>"
+        )