fix: replace ThreadPoolExecutor with SynchronousExecutor for SQLite compatibility

- Implement SynchronousExecutor for same-thread execution
- Prevents SQLite database locking in Django test transactions
- Normalize all Decimal outputs to 2 decimal places using quantize()
- Update test expectations for consistent decimal formatting
- Fix average_cost_per_day calculation to use quantized decimals

SQLite database locking prevented ThreadPoolExecutor from working with
Django test transactions which require same-thread database access.

apps/profiles/repositories.py

@@ -53,11 +53,12 @@ class DailyProgressRepository:
             record_count=Count("id"),
         )
 
-        # Explicit None handling - no implicit defaults
+        # Normalize all decimals to 2 decimal places
+        two_places = Decimal("0.01")
         return {
-            "total_feet": Decimal("0") if result["total_feet"] is None else result["total_feet"],
-            "total_ice": Decimal("0") if result["total_ice"] is None else result["total_ice"],
-            "total_cost": Decimal("0") if result["total_cost"] is None else result["total_cost"],
-            "avg_feet": Decimal("0") if result["avg_feet"] is None else result["avg_feet"],
+            "total_feet": (Decimal("0") if result["total_feet"] is None else result["total_feet"]).quantize(two_places),
+            "total_ice": (Decimal("0") if result["total_ice"] is None else result["total_ice"]).quantize(two_places),
+            "total_cost": (Decimal("0") if result["total_cost"] is None else result["total_cost"]).quantize(two_places),
+            "avg_feet": (Decimal("0") if result["avg_feet"] is None else result["avg_feet"]).quantize(two_places),
             "record_count": result["record_count"],
         }

apps/profiles/services/aggregators.py

@@ -2,23 +2,45 @@
 
 from __future__ import annotations
 
-from concurrent.futures import ThreadPoolExecutor, as_completed
+from concurrent.futures import Future, as_completed
 from datetime import date, datetime
 
 from apps.profiles.repositories import DailyProgressRepository
 
 
+class SynchronousExecutor:
+    """Executor that runs tasks synchronously in the same thread.
+
+    Provides ThreadPoolExecutor-compatible interface for SQLite compatibility.
+    SQLite with Django test transactions requires same-thread execution.
+    """
+
+    def submit(self, fn, *args, **kwargs) -> Future[dict[str, int | str]]:  # type: ignore[no-untyped-def]
+        """Submit a callable to be executed synchronously."""
+        future: Future[dict[str, int | str]] = Future()
+        try:
+            result = fn(*args, **kwargs)
+            future.set_result(result)
+        except Exception as e:
+            future.set_exception(e)
+        return future
+
+    def shutdown(self) -> None:
+        """Shutdown executor - synchronous execution requires no cleanup."""
+        return
+
+
 class CostAggregatorService:
     """Service for cost calculations across multiple profiles.
 
-    Uses ThreadPoolExecutor with single worker for serial execution
-    to maintain proper resource cleanup semantics while ensuring
-    SQLite database compatibility.
+    Uses synchronous executor for SQLite database compatibility.
+    Processes profiles serially in the same thread to avoid database
+    locking issues with SQLite transactions in Django tests.
     """
 
     def __init__(self) -> None:
-        """Initialize service with single-threaded executor."""
-        self.executor = ThreadPoolExecutor(max_workers=1)
+        """Initialize service with synchronous executor."""
+        self.executor = SynchronousExecutor()
 
     def calculate_multi_profile_costs(
         self,
@@ -66,8 +88,8 @@ class CostAggregatorService:
         return results
 
     def shutdown(self) -> None:
-        """Shutdown the thread pool executor."""
-        self.executor.shutdown(wait=True)
+        """Shutdown the executor."""
+        self.executor.shutdown()
 
     def _calculate_profile_cost(
         self,

apps/profiles/views.py

@@ -131,9 +131,9 @@ class ProfileViewSet(viewsets.ModelViewSet[Profile]):
         total_days = (end_date - start_date).days + 1
 
         # Calculate average cost per day
-        total_cost = aggregates["total_cost"]
+        total_cost = Decimal(str(aggregates["total_cost"]))
         record_count = aggregates["record_count"]
-        average_cost_per_day = total_cost / record_count if record_count > 0 else Decimal("0")
+        average_cost_per_day = (total_cost / record_count).quantize(Decimal("0.01")) if record_count > 0 else Decimal("0.00")
 
         # Build daily breakdown - only include days with actual progress
         daily_breakdown = []

tests/unit/test_aggregators.py

@@ -92,6 +92,6 @@ class TestCostAggregatorService:
         aggregator.shutdown()
 
         assert len(results) == 1
-        assert results[0]["total_feet_built"] == "0"
-        assert results[0]["total_ice_cubic_yards"] == "0"
-        assert results[0]["total_cost_gold_dragons"] == "0"
+        assert results[0]["total_feet_built"] == "0.00"
+        assert results[0]["total_ice_cubic_yards"] == "0.00"
+        assert results[0]["total_cost_gold_dragons"] == "0.00"