Make the GL account-code check report instead of refusing to start

The check shipped in f753cae threw on a missing code, which core-dumped
erpcore.service on deploy: 11 of the 12 configured Gl*Code values turned
out never to have existed in GL's chart of accounts, so a long-standing
mismatch became an immediate outage.

Detection was the point; refusing to serve the rest of the app was not.
GL already rejects a posting to an unknown account, so the per-flow error
stays the real enforcement — this now logs the whole set at Error once at
boot instead of surfacing one flow at a time.

Hard-fail is still available behind
GeneralLedgerService:FailOnMissingAccountCodes, for once the chart of
accounts is known-good and drift should stop a deploy.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Backend/ERPCore.Tests/GlAccountCodeValidatorTests.cs

@@ -26,8 +26,12 @@ public class GlAccountCodeValidatorTests
         await validator.StartAsync(CancellationToken.None);
     }
 
+    /// <summary>
+    /// The default must not block startup. Throwing here cost an outage on 2026-10-07 — a config
+    /// mismatch that had existed all along became a core-dump the moment the check shipped.
+    /// </summary>
     [Fact]
-    public async Task StartAsync_Throws_AndNamesEveryMissingCode()
+    public async Task StartAsync_DoesNotThrow_WhenCodesAreMissing_ByDefault()
     {
         var validator = CreateValidator(
             new()
@@ -36,6 +40,22 @@ public class GlAccountCodeValidatorTests
                 ["SalesDayEnd:GlCashAccountCode"] = "1000-01",
                 ["Adjustment:GlLossAccountCode"] = "5100"
             },
+            new StubGl(["1000-01"]));
+
+        await validator.StartAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task StartAsync_Throws_AndNamesEveryMissingCode_WhenConfiguredToFail()
+    {
+        var validator = CreateValidator(
+            new()
+            {
+                ["GeneralLedgerService:FailOnMissingAccountCodes"] = "true",
+                ["Grn:GlInventoryAccountCode"] = "1100",
+                ["SalesDayEnd:GlCashAccountCode"] = "1000-01",
+                ["Adjustment:GlLossAccountCode"] = "5100"
+            },
             new StubGl(["1100"]));
 
         var ex = await Assert.ThrowsAsync<InvalidOperationException>(
@@ -43,6 +63,7 @@ public class GlAccountCodeValidatorTests
 
         Assert.Contains("1000-01", ex.Message);
         Assert.Contains("5100", ex.Message);
+        Assert.Contains("2 of 3", ex.Message);
         Assert.DoesNotContain("1100,", ex.Message);
     }