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/sprint.py

@@ -0,0 +1,74 @@
+# app/models/syndarix/sprint.py
+"""
+Sprint model for Syndarix AI consulting platform.
+
+A Sprint represents a time-boxed iteration for organizing and delivering work.
+"""
+
+from sqlalchemy import Column, Date, Enum, ForeignKey, Index, Integer, String, Text
+from sqlalchemy.dialects.postgresql import UUID as PGUUID
+from sqlalchemy.orm import relationship
+
+from app.models.base import Base, TimestampMixin, UUIDMixin
+
+from .enums import SprintStatus
+
+
+class Sprint(Base, UUIDMixin, TimestampMixin):
+    """
+    Sprint model representing a time-boxed iteration.
+
+    Tracks:
+    - Sprint metadata (name, number, goal)
+    - Date range (start/end)
+    - Progress metrics (planned vs completed points)
+    """
+
+    __tablename__ = "sprints"
+
+    # Foreign key to project
+    project_id = Column(
+        PGUUID(as_uuid=True),
+        ForeignKey("projects.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+
+    # Sprint identification
+    name = Column(String(255), nullable=False)
+    number = Column(Integer, nullable=False)  # Sprint number within project
+
+    # Sprint goal (what we aim to achieve)
+    goal = Column(Text, nullable=True)
+
+    # Date range
+    start_date = Column(Date, nullable=False, index=True)
+    end_date = Column(Date, nullable=False, index=True)
+
+    # Status
+    status: Column[SprintStatus] = Column(
+        Enum(SprintStatus),
+        default=SprintStatus.PLANNED,
+        nullable=False,
+        index=True,
+    )
+
+    # Progress metrics
+    planned_points = Column(Integer, nullable=True)  # Sum of story points at start
+    completed_points = Column(Integer, nullable=True)  # Sum of completed story points
+
+    # Relationships
+    project = relationship("Project", back_populates="sprints")
+    issues = relationship("Issue", back_populates="sprint")
+
+    __table_args__ = (
+        Index("ix_sprints_project_status", "project_id", "status"),
+        Index("ix_sprints_project_number", "project_id", "number"),
+        Index("ix_sprints_date_range", "start_date", "end_date"),
+    )
+
+    def __repr__(self) -> str:
+        return (
+            f"<Sprint {self.name} (#{self.number}) "
+            f"project={self.project_id} status={self.status.value}>"
+        )