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/schemas.py

@@ -182,3 +182,38 @@ class TargetRead(BaseModel):
     protein_g: float | None
     carbs_g: float | None
     fat_g: float | None
+
+
+# ── Day Summary (TICKET-004) ────────────────────────────────────────────────
+
+
+class DaySummaryNutrition(BaseModel):
+    """Summed nutrition totals for a single day.
+
+    All fields are floats (summed from scaled per-unit values).
+    Foods with null nutrition fields contribute 0.
+    """
+
+    calories: float
+    protein_g: float
+    carbs_g: float
+    fat_g: float
+    fiber_g: float
+    saturated_fat_g: float
+    sugars_g: float
+    sodium_g: float
+
+
+class DaySummaryResponse(BaseModel):
+    """Response shape for GET /api/log/summary — the contract the frontend
+    ProgressBar consumes.
+
+    Remaining-vs-target arithmetic: LEFT TO THE FRONTEND.
+    The server returns raw totals and the applicable target (or null).
+    The frontend computes remaining = target.calories - totals.calories
+    and similar for macros where both target and total exist.
+    """
+
+    date: date
+    totals: DaySummaryNutrition
+    target: TargetRead | None