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,
+    )