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/sprint.py

@@ -0,0 +1,135 @@
+# app/schemas/syndarix/sprint.py
+"""
+Pydantic schemas for Sprint entity.
+"""
+
+from datetime import date, datetime
+from uuid import UUID
+
+from pydantic import BaseModel, ConfigDict, Field, field_validator, model_validator
+
+from .enums import SprintStatus
+
+
+class SprintBase(BaseModel):
+    """Base sprint schema with common fields."""
+
+    name: str = Field(..., min_length=1, max_length=255)
+    number: int = Field(..., ge=1)
+    goal: str | None = None
+    start_date: date
+    end_date: date
+    status: SprintStatus = SprintStatus.PLANNED
+    planned_points: int | None = Field(None, ge=0)
+    completed_points: int | None = Field(None, ge=0)
+
+    @field_validator("name")
+    @classmethod
+    def validate_name(cls, v: str) -> str:
+        """Validate sprint name."""
+        if not v or v.strip() == "":
+            raise ValueError("Sprint name cannot be empty")
+        return v.strip()
+
+    @model_validator(mode="after")
+    def validate_dates(self) -> "SprintBase":
+        """Validate that end_date is after start_date."""
+        if self.end_date < self.start_date:
+            raise ValueError("End date must be after or equal to start date")
+        return self
+
+
+class SprintCreate(SprintBase):
+    """Schema for creating a new sprint."""
+
+    project_id: UUID
+
+
+class SprintUpdate(BaseModel):
+    """Schema for updating a sprint."""
+
+    name: str | None = Field(None, min_length=1, max_length=255)
+    goal: str | None = None
+    start_date: date | None = None
+    end_date: date | None = None
+    status: SprintStatus | None = None
+    planned_points: int | None = Field(None, ge=0)
+    completed_points: int | None = Field(None, ge=0)
+
+    @field_validator("name")
+    @classmethod
+    def validate_name(cls, v: str | None) -> str | None:
+        """Validate sprint name."""
+        if v is not None and (not v or v.strip() == ""):
+            raise ValueError("Sprint name cannot be empty")
+        return v.strip() if v else v
+
+
+class SprintStart(BaseModel):
+    """Schema for starting a sprint."""
+
+    start_date: date | None = None  # Optionally override start date
+
+
+class SprintComplete(BaseModel):
+    """Schema for completing a sprint."""
+
+    completed_points: int | None = Field(None, ge=0)
+    notes: str | None = None
+
+
+class SprintInDB(SprintBase):
+    """Schema for sprint in database."""
+
+    id: UUID
+    project_id: UUID
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class SprintResponse(SprintBase):
+    """Schema for sprint API responses."""
+
+    id: UUID
+    project_id: UUID
+    created_at: datetime
+    updated_at: datetime
+
+    # Expanded fields from relationships
+    project_name: str | None = None
+    project_slug: str | None = None
+    issue_count: int | None = 0
+    open_issues: int | None = 0
+    completed_issues: int | None = 0
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class SprintListResponse(BaseModel):
+    """Schema for paginated sprint list responses."""
+
+    sprints: list[SprintResponse]
+    total: int
+    page: int
+    page_size: int
+    pages: int
+
+
+class SprintVelocity(BaseModel):
+    """Schema for sprint velocity metrics."""
+
+    sprint_number: int
+    sprint_name: str
+    planned_points: int | None
+    completed_points: int | None
+    velocity: float | None  # completed/planned ratio
+
+
+class SprintBurndown(BaseModel):
+    """Schema for sprint burndown data point."""
+
+    date: date
+    remaining_points: int
+    ideal_remaining: float