TICKET-003: Daily log write path

- POST/PUT/DELETE /api/log, GET /api/log?date= with embedded food
- sort_order auto-appends per day; hard delete for log entries
- Soft-deleted foods rejected for new logs (404) but render in history
- Full suite green (87 passed)

backend/services/log.py

@@ -0,0 +1,127 @@
+"""Daily log service layer — business logic + DB access (spec §8.1 rules 2–3).
+
+- Log entries reference foods; embedded food data is included in responses.
+- sort_order for new entries defaults to end of day: max(sort_order) + 1
+  for that date, starting at 1 if no entries exist yet.
+- Soft-deleted foods are rejected on create (404 — you can't log what you
+  can't search), but still render in historical GET responses (§2.1).
+
+ORM objects never leave this module; functions return Pydantic schemas.
+"""
+
+from datetime import date, datetime, timezone
+
+from sqlalchemy import func, select
+from sqlalchemy.orm import Session, joinedload
+
+from models import DailyLogEntry, Food
+from schemas import LogEntryCreate, LogEntryRead, LogEntryUpdate
+
+
+def _now() -> datetime:
+    return datetime.now(timezone.utc).replace(tzinfo=None)
+
+
+def _load_entry_with_food(db: Session, entry_id: int) -> DailyLogEntry | None:
+    """Eager-load a single entry with its food relationship."""
+    return db.scalar(
+        select(DailyLogEntry)
+        .where(DailyLogEntry.id == entry_id)
+        .options(joinedload(DailyLogEntry.food))
+    )
+
+
+# ── CRUD ─────────────────────────────────────────────────────────────────────
+
+
+def get_log_entries(db: Session, lookup_date: date) -> list[LogEntryRead]:
+    """All entries for a date, ordered by sort_order then id, with embedded
+    food data eagerly loaded. Soft-deleted foods still render (§2.1)."""
+    stmt = (
+        select(DailyLogEntry)
+        .where(DailyLogEntry.date == lookup_date)
+        .options(joinedload(DailyLogEntry.food))
+        .order_by(DailyLogEntry.sort_order, DailyLogEntry.id)
+    )
+    entries = db.scalars(stmt).all()
+    return [LogEntryRead.model_validate(e) for e in entries]
+
+
+def create_log_entry(db: Session, data: LogEntryCreate) -> LogEntryRead:
+    """Create a log entry.
+
+    Validates that food_id exists and is not soft-deleted.
+    sort_order defaults to end of day for the given date.
+
+    Raises FoodNotAvailableError if food_id is unknown or the food is
+    soft-deleted (you can't log what you can't search — spec §2.1).
+    """
+    food = db.get(Food, data.food_id)
+    if food is None or food.deleted_at is not None:
+        raise FoodNotAvailableError(data.food_id)
+
+    max_sort = db.scalar(
+        select(func.max(DailyLogEntry.sort_order)).where(
+            DailyLogEntry.date == data.date
+        )
+    )
+    sort_order = (max_sort or 0) + 1
+
+    entry = DailyLogEntry(
+        date=data.date,
+        food_id=data.food_id,
+        quantity=data.quantity,
+        meal_slot=data.meal_slot,
+        sort_order=sort_order,
+        created_at=_now(),
+    )
+    db.add(entry)
+    db.commit()
+
+    # Re-query with eager-loaded food for the response
+    return LogEntryRead.model_validate(_load_entry_with_food(db, entry.id))
+
+
+def update_log_entry(
+    db: Session, entry_id: int, data: LogEntryUpdate
+) -> LogEntryRead | None:
+    """Update quantity, meal_slot, and/or sort_order.
+
+    Uses model_dump(exclude_unset=True) so only explicitly-provided fields
+    are changed. Send {"meal_slot": null} to clear the slot.
+
+    Returns None if the entry is not found.
+    """
+    entry = db.get(DailyLogEntry, entry_id)
+    if entry is None:
+        return None
+
+    update_data = data.model_dump(exclude_unset=True)
+    for field, value in update_data.items():
+        setattr(entry, field, value)
+
+    db.commit()
+    return LogEntryRead.model_validate(_load_entry_with_food(db, entry_id))
+
+
+def delete_log_entry(db: Session, entry_id: int) -> bool:
+    """Hard-delete a log entry (§2.3 has no soft-delete).
+
+    Returns True if deleted, False if not found.
+    """
+    entry = db.get(DailyLogEntry, entry_id)
+    if entry is None:
+        return False
+    db.delete(entry)
+    db.commit()
+    return True
+
+
+# ── Errors ───────────────────────────────────────────────────────────────────
+
+
+class FoodNotAvailableError(Exception):
+    """Raised when a food_id is unknown or the food is soft-deleted."""
+    def __init__(self, food_id: int):
+        super().__init__(f"Food {food_id} not available for logging")
+        self.food_id = food_id