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/issue.py

@@ -0,0 +1,133 @@
+# app/models/syndarix/issue.py
+"""
+Issue model for Syndarix AI consulting platform.
+
+An Issue represents a unit of work that can be assigned to agents or humans,
+with optional synchronization to external issue trackers (Gitea, GitHub, GitLab).
+"""
+
+from sqlalchemy import Column, DateTime, Enum, ForeignKey, Index, Integer, 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 IssuePriority, IssueStatus, SyncStatus
+
+
+class Issue(Base, UUIDMixin, TimestampMixin):
+    """
+    Issue model representing a unit of work in a project.
+
+    Features:
+    - Standard issue fields (title, body, status, priority)
+    - Assignment to agent instances or human assignees
+    - Sprint association for backlog management
+    - External tracker synchronization (Gitea, GitHub, GitLab)
+    """
+
+    __tablename__ = "issues"
+
+    # Foreign key to project
+    project_id = Column(
+        PGUUID(as_uuid=True),
+        ForeignKey("projects.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+
+    # Issue content
+    title = Column(String(500), nullable=False)
+    body = Column(Text, nullable=False, default="")
+
+    # Status and priority
+    status: Column[IssueStatus] = Column(
+        Enum(IssueStatus),
+        default=IssueStatus.OPEN,
+        nullable=False,
+        index=True,
+    )
+
+    priority: Column[IssuePriority] = Column(
+        Enum(IssuePriority),
+        default=IssuePriority.MEDIUM,
+        nullable=False,
+        index=True,
+    )
+
+    # Labels for categorization (e.g., ["bug", "frontend", "urgent"])
+    labels = Column(JSONB, default=list, nullable=False)
+
+    # Assignment - either to an agent or a human (mutually exclusive)
+    assigned_agent_id = Column(
+        PGUUID(as_uuid=True),
+        ForeignKey("agent_instances.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
+
+    # Human assignee (username or email, not a FK to allow external users)
+    human_assignee = Column(String(255), nullable=True, index=True)
+
+    # Sprint association
+    sprint_id = Column(
+        PGUUID(as_uuid=True),
+        ForeignKey("sprints.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
+
+    # Story points for estimation
+    story_points = Column(Integer, nullable=True)
+
+    # External tracker integration
+    external_tracker = Column(
+        String(50),
+        nullable=True,
+        index=True,
+    )  # 'gitea', 'github', 'gitlab'
+
+    external_id = Column(String(255), nullable=True)  # External system's ID
+    external_url = Column(String(1000), nullable=True)  # Link to external issue
+    external_number = Column(Integer, nullable=True)  # Issue number (e.g., #123)
+
+    # Sync status with external tracker
+    sync_status: Column[SyncStatus] = Column(
+        Enum(SyncStatus),
+        default=SyncStatus.SYNCED,
+        nullable=False,
+        # Note: Index defined in __table_args__ as ix_issues_sync_status
+    )
+
+    last_synced_at = Column(DateTime(timezone=True), nullable=True)
+    external_updated_at = Column(DateTime(timezone=True), nullable=True)
+
+    # Lifecycle timestamp
+    closed_at = Column(DateTime(timezone=True), nullable=True, index=True)
+
+    # Relationships
+    project = relationship("Project", back_populates="issues")
+    assigned_agent = relationship(
+        "AgentInstance",
+        back_populates="assigned_issues",
+        foreign_keys=[assigned_agent_id],
+    )
+    sprint = relationship("Sprint", back_populates="issues")
+
+    __table_args__ = (
+        Index("ix_issues_project_status", "project_id", "status"),
+        Index("ix_issues_project_priority", "project_id", "priority"),
+        Index("ix_issues_project_sprint", "project_id", "sprint_id"),
+        Index("ix_issues_external_tracker_id", "external_tracker", "external_id"),
+        Index("ix_issues_sync_status", "sync_status"),
+        Index("ix_issues_project_agent", "project_id", "assigned_agent_id"),
+    )
+
+    def __repr__(self) -> str:
+        return (
+            f"<Issue {self.id} title='{self.title[:30]}...' "
+            f"status={self.status.value} priority={self.priority.value}>"
+        )