Fix MariaDB CHECK constraint quoting that crash-looped startup seeding

Three CHECK constraints shipped with PostgreSQL identifier quoting left over
from the MySQL port:

  ck_grn_config_singleton       CHECK ("ConfigId" = 1)
  ck_sales_config_singleton     CHECK ("ConfigId" = 1)
  ck_warranty_periods_months    CHECK ("Months" BETWEEN 1 AND 600)

ANSI_QUOTES is not in MariaDB's default sql_mode, so a double-quoted token is a
*string literal*, not an identifier. `"ConfigId" = 1` therefore evaluates as
`'ConfigId' = 1`; MariaDB coerces the string to a number to compare it and,
under strict mode, raises `Truncated incorrect DECIMAL value: 'ConfigId'`.

The DDL creates fine and only breaks at runtime, so this got through the
deploy and then failed every INSERT into grn_config, sales_config and
warranty_periods. That includes DataSeeder's two singleton config rows, which
run immediately after MigrateAsync() in Program.cs — so the service threw
"Database migration succeeded, but startup seeding failed" on every boot and
systemd restart-looped (counter reached 110, each restart dumping core).

Fixed in the entity configurations and repaired on existing databases by
migration 20261005190000_FixCheckConstraintQuotingForMySql, which drops and
re-adds the three constraints with backticks. The drop/add is emitted as raw
SQL rather than DropCheckConstraint because MariaDB spells the removal
DROP CONSTRAINT, not MySQL's DROP CHECK; IF EXISTS keeps it re-runnable on a
database where the broken constraint was already removed by hand.

The identical trap was swept during the port itself; these three were
reintroduced by a feature migration written afterwards.

No data cleanup is needed: the failing inserts never committed, so the
affected tables are empty.

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

Backend/ERPCore/Infra/Persistence/Configurations/GrnConfigConfiguration.cs

@@ -13,7 +13,7 @@ public sealed class GrnConfigConfiguration : IEntityTypeConfiguration<GrnConfig>
     public void Configure(EntityTypeBuilder<GrnConfig> builder)
     {
         builder.ToTable("grn_config", t =>
-            t.HasCheckConstraint("ck_grn_config_singleton", $"\"ConfigId\" = {GrnConfig.SingletonId}"));
+            t.HasCheckConstraint("ck_grn_config_singleton", $"`ConfigId` = {GrnConfig.SingletonId}"));
 
         builder.HasKey(c => c.ConfigId);
 

Backend/ERPCore/Infra/Persistence/Configurations/SalesConfigConfiguration.cs

@@ -13,7 +13,7 @@ public sealed class SalesConfigConfiguration : IEntityTypeConfiguration<SalesCon
     public void Configure(EntityTypeBuilder<SalesConfig> builder)
     {
         builder.ToTable("sales_config", t =>
-            t.HasCheckConstraint("ck_sales_config_singleton", $"\"ConfigId\" = {SalesConfig.SingletonId}"));
+            t.HasCheckConstraint("ck_sales_config_singleton", $"`ConfigId` = {SalesConfig.SingletonId}"));
 
         builder.HasKey(c => c.ConfigId);
 

Backend/ERPCore/Infra/Persistence/Configurations/WarrantyPeriodConfiguration.cs

@@ -11,7 +11,7 @@ public sealed class WarrantyPeriodConfiguration : IEntityTypeConfiguration<Warra
     public void Configure(EntityTypeBuilder<WarrantyPeriod> builder)
     {
         builder.ToTable("warranty_periods", t =>
-            t.HasCheckConstraint("ck_warranty_periods_months", "\"Months\" BETWEEN 1 AND 600"));
+            t.HasCheckConstraint("ck_warranty_periods_months", "`Months` BETWEEN 1 AND 600"));
         builder.HasKey(p => p.WarrantyPeriodId);
 
         builder.Property(p => p.Months).IsRequired();