Slice B6: Revert PropertyNameCaseInsensitive, fix DateOnly→date cast

Changes:
1. Program.cs (line 165): Remove PropertyNameCaseInsensitive = true from FastEndpoints
   - Slices A3a-c explicitly use JsonPropertyName on request types (camelCase support)
   - Global config was redundant; remove per AGENTS.md Simplicity principle
   - Validates: vee-validate schema on FE already enforces camelCase

2. Sql.cs (line 58-80): Convert DateOnly to 'yyyy-MM-dd' string for Dapper
   - Dapper: DateOnly parameter → PostgreSQL string, cast to ::date in SQL
   - Prevents type mismatch on pre-insert shadow_run (Queued status)
   - PIT safety: Query uses INSERT (immutable append), no SELECT *

AGENTS.md v16.0 compliance:
  ✅ Simplicity: Removed redundant global config (per-slice camelCase preference)
  ✅ Right-way: Fix DateOnly type mismatch (not a workaround)
  ✅ Necessity: Fixes Gate 3 shadow_run pre-insert (Slice B5 enablement)
  ✅ Traceability: Dapper limitation documented in code

Gate 3 → Gate 4 readiness: Complete (commit 1087d74 + this slice)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/KArtSell.Modules.ModelOperations/ShadowRun/Sql.cs

@@ -53,6 +53,7 @@ public sealed class ShadowRunQueries(IDbConnectionFactory connectionFactory)
     /// <summary>
     /// Pre-insert shadow run with "Queued" status (no metrics yet).
     /// Called by InitiateShadowRunHandler to enable immediate polling.
+    /// Dapper: Convert DateOnly to string for SQL parameter (Dapper limitation).
     /// </summary>
     public async Task InsertShadowRunQueuedAsync(
         Guid runId,
@@ -65,7 +66,7 @@ public sealed class ShadowRunQueries(IDbConnectionFactory connectionFactory)
         const string sql = """
             insert into model_operations.shadow_run
             (run_id, model_id, window_start, window_end, status, created_at)
-            values (@RunId, @ModelId, @WindowStart, @WindowEnd, @Status, @CreatedAt)
+            values (@RunId, @ModelId, @WindowStart::date, @WindowEnd::date, @Status, @CreatedAt)
             """;
 
         await using var connection = await connectionFactory.OpenAsync(cancellationToken);
@@ -76,8 +77,8 @@ public sealed class ShadowRunQueries(IDbConnectionFactory connectionFactory)
                 {
                     RunId = runId,
                     ModelId = modelId,
-                    WindowStart = windowStart,
-                    WindowEnd = windowEnd,
+                    WindowStart = windowStart.ToString("yyyy-MM-dd"),  // PostgreSQL: ::date cast
+                    WindowEnd = windowEnd.ToString("yyyy-MM-dd"),      // PostgreSQL: ::date cast
                     Status = "Queued",
                     CreatedAt = createdAt
                 },