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/issue.py

@@ -0,0 +1,193 @@
+# app/schemas/syndarix/issue.py
+"""
+Pydantic schemas for Issue entity.
+"""
+
+from datetime import datetime
+from typing import Literal
+from uuid import UUID
+
+from pydantic import BaseModel, ConfigDict, Field, field_validator, model_validator
+
+from .enums import IssuePriority, IssueStatus, SyncStatus
+
+
+class IssueBase(BaseModel):
+    """Base issue schema with common fields."""
+
+    title: str = Field(..., min_length=1, max_length=500)
+    body: str = ""
+    status: IssueStatus = IssueStatus.OPEN
+    priority: IssuePriority = IssuePriority.MEDIUM
+    labels: list[str] = Field(default_factory=list)
+    story_points: int | None = Field(None, ge=0, le=100)
+
+    @field_validator("title")
+    @classmethod
+    def validate_title(cls, v: str) -> str:
+        """Validate issue title."""
+        if not v or v.strip() == "":
+            raise ValueError("Issue title cannot be empty")
+        return v.strip()
+
+    @field_validator("labels")
+    @classmethod
+    def validate_labels(cls, v: list[str]) -> list[str]:
+        """Validate and normalize labels."""
+        return [label.strip().lower() for label in v if label.strip()]
+
+
+class IssueCreate(IssueBase):
+    """Schema for creating a new issue."""
+
+    project_id: UUID
+    assigned_agent_id: UUID | None = None
+    human_assignee: str | None = Field(None, max_length=255)
+    sprint_id: UUID | None = None
+
+    # External tracker fields (optional, for importing from external systems)
+    external_tracker: Literal["gitea", "github", "gitlab"] | None = None
+    external_id: str | None = Field(None, max_length=255)
+    external_url: str | None = Field(None, max_length=1000)
+    external_number: int | None = None
+
+
+class IssueUpdate(BaseModel):
+    """Schema for updating an issue."""
+
+    title: str | None = Field(None, min_length=1, max_length=500)
+    body: str | None = None
+    status: IssueStatus | None = None
+    priority: IssuePriority | None = None
+    labels: list[str] | None = None
+    assigned_agent_id: UUID | None = None
+    human_assignee: str | None = Field(None, max_length=255)
+    sprint_id: UUID | None = None
+    story_points: int | None = Field(None, ge=0, le=100)
+    sync_status: SyncStatus | None = None
+
+    @field_validator("title")
+    @classmethod
+    def validate_title(cls, v: str | None) -> str | None:
+        """Validate issue title."""
+        if v is not None and (not v or v.strip() == ""):
+            raise ValueError("Issue title cannot be empty")
+        return v.strip() if v else v
+
+    @field_validator("labels")
+    @classmethod
+    def validate_labels(cls, v: list[str] | None) -> list[str] | None:
+        """Validate and normalize labels."""
+        if v is None:
+            return v
+        return [label.strip().lower() for label in v if label.strip()]
+
+
+class IssueClose(BaseModel):
+    """Schema for closing an issue."""
+
+    resolution: str | None = None  # Optional resolution note
+
+
+class IssueAssign(BaseModel):
+    """Schema for assigning an issue."""
+
+    assigned_agent_id: UUID | None = None
+    human_assignee: str | None = Field(None, max_length=255)
+
+    @model_validator(mode="after")
+    def validate_assignment(self) -> "IssueAssign":
+        """Ensure only one type of assignee is set."""
+        if self.assigned_agent_id and self.human_assignee:
+            raise ValueError(
+                "Cannot assign to both an agent and a human. Choose one."
+            )
+        return self
+
+
+class IssueSyncUpdate(BaseModel):
+    """Schema for updating sync-related fields."""
+
+    sync_status: SyncStatus
+    last_synced_at: datetime | None = None
+    external_updated_at: datetime | None = None
+
+
+class IssueInDB(IssueBase):
+    """Schema for issue in database."""
+
+    id: UUID
+    project_id: UUID
+    assigned_agent_id: UUID | None = None
+    human_assignee: str | None = None
+    sprint_id: UUID | None = None
+    external_tracker: str | None = None
+    external_id: str | None = None
+    external_url: str | None = None
+    external_number: int | None = None
+    sync_status: SyncStatus = SyncStatus.SYNCED
+    last_synced_at: datetime | None = None
+    external_updated_at: datetime | None = None
+    closed_at: datetime | None = None
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class IssueResponse(BaseModel):
+    """Schema for issue API responses."""
+
+    id: UUID
+    project_id: UUID
+    title: str
+    body: str
+    status: IssueStatus
+    priority: IssuePriority
+    labels: list[str] = Field(default_factory=list)
+    assigned_agent_id: UUID | None = None
+    human_assignee: str | None = None
+    sprint_id: UUID | None = None
+    story_points: int | None = None
+    external_tracker: str | None = None
+    external_id: str | None = None
+    external_url: str | None = None
+    external_number: int | None = None
+    sync_status: SyncStatus = SyncStatus.SYNCED
+    last_synced_at: datetime | None = None
+    external_updated_at: datetime | None = None
+    closed_at: datetime | None = None
+    created_at: datetime
+    updated_at: datetime
+
+    # Expanded fields from relationships
+    project_name: str | None = None
+    project_slug: str | None = None
+    sprint_name: str | None = None
+    assigned_agent_type_name: str | None = None
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class IssueListResponse(BaseModel):
+    """Schema for paginated issue list responses."""
+
+    issues: list[IssueResponse]
+    total: int
+    page: int
+    page_size: int
+    pages: int
+
+
+class IssueStats(BaseModel):
+    """Schema for issue statistics."""
+
+    total: int
+    open: int
+    in_progress: int
+    in_review: int
+    blocked: int
+    closed: int
+    by_priority: dict[str, int]
+    total_story_points: int | None = None
+    completed_story_points: int | None = None