Fix Reports 400 errors: normalize DateTime to UTC and restore camelCase JSON

Query-bound DateTimes have Kind=Unspecified which Npgsql rejects for
timestamptz columns, causing InvalidOperationException caught as 400.
Also add explicit CamelCase naming policy lost when AddJsonOptions was
introduced for the enum converter.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/Purrse.Api/Controllers/ReportsController.cs

@@ -18,17 +18,20 @@ public class ReportsController : ControllerBase
 
     [HttpGet("spending-by-category")]
     public async Task<ActionResult<SpendingByCategoryReport>> SpendingByCategory([FromQuery] DateTime startDate, [FromQuery] DateTime endDate, [FromQuery] Guid? accountId)
-        => Ok(await _reportService.GetSpendingByCategoryAsync(UserId, startDate, endDate, accountId));
+        => Ok(await _reportService.GetSpendingByCategoryAsync(UserId, Utc(startDate), Utc(endDate), accountId));
 
     [HttpGet("income-vs-expense")]
     public async Task<ActionResult<IncomeVsExpenseReport>> IncomeVsExpense([FromQuery] DateTime startDate, [FromQuery] DateTime endDate)
-        => Ok(await _reportService.GetIncomeVsExpenseAsync(UserId, startDate, endDate));
+        => Ok(await _reportService.GetIncomeVsExpenseAsync(UserId, Utc(startDate), Utc(endDate)));
 
     [HttpGet("net-worth")]
     public async Task<ActionResult<NetWorthReport>> NetWorth([FromQuery] DateTime startDate, [FromQuery] DateTime endDate)
-        => Ok(await _reportService.GetNetWorthAsync(UserId, startDate, endDate));
+        => Ok(await _reportService.GetNetWorthAsync(UserId, Utc(startDate), Utc(endDate)));
 
     [HttpGet("cash-flow")]
     public async Task<ActionResult<CashFlowReport>> CashFlow([FromQuery] DateTime startDate, [FromQuery] DateTime endDate)
-        => Ok(await _reportService.GetCashFlowAsync(UserId, startDate, endDate));
+        => Ok(await _reportService.GetCashFlowAsync(UserId, Utc(startDate), Utc(endDate)));
+
+    private static DateTime Utc(DateTime dt) =>
+        dt.Kind == DateTimeKind.Unspecified ? DateTime.SpecifyKind(dt, DateTimeKind.Utc) : dt;
 }

src/Purrse.Api/Program.cs

@@ -105,6 +105,7 @@ builder.Services.AddSignalR();
 builder.Services.AddControllers()
     .AddJsonOptions(options =>
     {
+        options.JsonSerializerOptions.PropertyNamingPolicy = System.Text.Json.JsonNamingPolicy.CamelCase;
         options.JsonSerializerOptions.Converters.Add(new System.Text.Json.Serialization.JsonStringEnumConverter());
     });
 builder.Services.AddEndpointsApiExplorer();