feat(backend): add Pydantic schemas

backend/src/schemas/stats.py

@@ -0,0 +1,34 @@
+from pydantic import BaseModel
+
+
+class DifficultyCount(BaseModel):
+    """Count of questions by difficulty."""
+
+    easy: int
+    medium: int
+    hard: int
+
+
+class CategoryCount(BaseModel):
+    """Question count for a category."""
+
+    name: str
+    slug: str
+    count: int
+
+
+class PatternCount(BaseModel):
+    """Question count for a pattern."""
+
+    name: str
+    slug: str
+    count: int
+
+
+class StatsResponse(BaseModel):
+    """Overall statistics response."""
+
+    total_questions: int
+    by_difficulty: DifficultyCount
+    by_category: list[CategoryCount]
+    by_pattern: list[PatternCount]