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.Host/Program.cs

@@ -161,10 +161,7 @@ builder.Services.AddAuthorization();
 builder.Services.AddSignalR();
 builder.Services.AddSignalEngineModule();
 builder.Services.AddModelOperationsModule();
-builder.Services.AddFastEndpoints(config =>
-{
-    config.SerializerOptions.PropertyNameCaseInsensitive = true;
-});  // AFTER modules registered (so their endpoints are included)
+builder.Services.AddFastEndpoints();  // AFTER modules registered (so their endpoints are included)
 builder.Services.AddEndpointsApiExplorer();
 builder.Services.AddSwaggerGen();