fix(backend): add unique constraint for sprint numbers

Add UniqueConstraint to Sprint model to ensure sprint numbers
are unique within a project, matching the migration specification.

🤖 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

@@ -5,7 +5,7 @@ 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 import Column, Date, Enum, ForeignKey, Index, Integer, String, Text, UniqueConstraint
 from sqlalchemy.dialects.postgresql import UUID as PGUUID
 from sqlalchemy.orm import relationship
 
@@ -65,6 +65,8 @@ class Sprint(Base, UUIDMixin, TimestampMixin):
         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"),
+        # Ensure sprint numbers are unique within a project
+        UniqueConstraint("project_id", "number", name="uq_sprint_project_number"),
     )
 
     def __repr__(self) -> str: