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/log.py

@@ -15,7 +15,15 @@ from sqlalchemy import func, select
 from sqlalchemy.orm import Session, joinedload
 
 from models import DailyLogEntry, Food
-from schemas import LogEntryCreate, LogEntryRead, LogEntryUpdate
+from schemas import (
+    DaySummaryNutrition,
+    DaySummaryResponse,
+    LogEntryCreate,
+    LogEntryRead,
+    LogEntryUpdate,
+)
+from services import nutrition
+from services import targets as targets_svc
 
 
 def _now() -> datetime:
@@ -125,3 +133,49 @@ class FoodNotAvailableError(Exception):
     def __init__(self, food_id: int):
         super().__init__(f"Food {food_id} not available for logging")
         self.food_id = food_id
+
+
+# ── Day Summary (TICKET-004) ─────────────────────────────────────────────────
+
+
+def get_day_summary(db: Session, lookup_date: date) -> DaySummaryResponse:
+    """Compute nutrition totals for a date vs. the applicable target.
+
+    1. Loads all log entries for the date with food data eagerly joined.
+    2. For each entry, scales the food's per-unit nutrition to the logged
+       quantity via nutrition.entry_nutrition() — weight-type foods get
+       (qty/100)× scaling, count-type foods get qty× scaling (§2.1).
+    3. Meal entries (is_meal=True) currently contribute 0 because their
+       per_unit fields are null per the CHECK constraint.
+       TODO: TICKET-007 — replace with recursive component summation.
+    4. Looks up the target covering the date via the half-open interval
+       lookup from targets.get_target_for_date(). Returns null if none.
+
+    Returns a DaySummaryResponse with summed totals and the applicable
+    target (or null).
+    """
+    entries = db.scalars(
+        select(DailyLogEntry)
+        .where(DailyLogEntry.date == lookup_date)
+        .options(joinedload(DailyLogEntry.food))
+    ).all()
+
+    # Sum nutrition across all entries for the date.
+    # TODO: TICKET-007 — meal entries currently contribute 0 because their
+    # per_unit fields are null per the CHECK constraint. Real meal nutrition
+    # will be derived by recursively summing component foods' nutrition.
+    # When that lands, replace the flat entry_nutrition() call with a
+    # meal-aware sum function from services/nutrition.py.
+    totals = {field: 0.0 for field in nutrition.NUTRITION_FIELDS}
+    for entry in entries:
+        entry_nut = nutrition.entry_nutrition(entry.food, entry.quantity)
+        for field, value in entry_nut.items():
+            totals[field] += value
+
+    target = targets_svc.get_target_for_date(db, lookup_date)
+
+    return DaySummaryResponse(
+        date=lookup_date,
+        totals=DaySummaryNutrition(**totals),
+        target=target,
+    )

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
+    }