TICKET-004: Day summary endpoint

- GET /api/log/summary?date= with totals vs historical target
- Nutrition math consolidated in services/nutrition.py (weight vs count)
- Null nutrition contributes 0; meals contribute 0 (TODO TICKET-007)
- Full suite green (104 passed)

backend/services/nutrition.py

@@ -9,6 +9,27 @@ Routers never compute nutrition; the frontend never re-derives it.
 
 from models import Food
 
+# All nutrition fields tracked on foods. Each has a corresponding *_per_unit
+# column on the Food model. Used by entry_nutrition() to iterate over fields.
+NUTRITION_FIELDS = [
+    "calories", "protein_g", "carbs_g", "fat_g",
+    "fiber_g", "saturated_fat_g", "sugars_g", "sodium_g",
+]
+
+# Maps each NUTRITION_FIELDS key to the Food model's per_unit column name.
+# Naming is slightly irregular (e.g. "protein_g" → "protein_per_unit",
+# not "protein_g_per_unit").
+_FIELD_TO_PER_UNIT_COL = {
+    "calories": "calories_per_unit",
+    "protein_g": "protein_per_unit",
+    "carbs_g": "carbs_per_unit",
+    "fat_g": "fat_per_unit",
+    "fiber_g": "fiber_per_unit",
+    "saturated_fat_g": "saturated_fat_per_unit",
+    "sugars_g": "sugars_per_unit",
+    "sodium_g": "sodium_per_unit",
+}
+
 
 def scale_to_quantity(per_unit: float | None, quantity: float, unit_type: str) -> float:
     """Scale a per-unit nutrition value to a logged quantity. Missing values count as 0."""
@@ -28,3 +49,24 @@ def entry_calories(food: Food, quantity: float) -> float:
     (recursively, with cycle detection — spec §2.2).
     """
     return scale_to_quantity(food.calories_per_unit, quantity, food.unit_type)
+
+
+def entry_nutrition(food: Food, quantity: float) -> dict[str, float]:
+    """Return all nutrition fields scaled to a logged quantity.
+
+    Each field is resolved via scale_to_quantity using the food's unit_type.
+    NULL per-unit values contribute 0.0, not an error.
+
+    Meal foods (is_meal=True) have null per_unit fields per the CHECK
+    constraint, so they naturally contribute 0 for all fields.
+    TODO: TICKET-007 — real meal nutrition will sum scaled component
+    foods recursively. Until then, meal entries contribute 0.
+    """
+    return {
+        field: scale_to_quantity(
+            getattr(food, _FIELD_TO_PER_UNIT_COL[field], None),
+            quantity,
+            food.unit_type,
+        )
+        for field in NUTRITION_FIELDS
+    }