ARCH-2: swap EF provider to Npgsql, drop SqlServer migrations + conn strings

- Microsoft.EntityFrameworkCore.SqlServer -> Npgsql.EntityFrameworkCore.PostgreSQL 10.0.2
- UseSqlServer -> UseNpgsql (Aspire-injected gerbilmanager connection)
- Removed hardcoded sa connection strings from appsettings*.json
- Deleted SqlServer-flavored initial migration (fresh Npgsql one follows; no prod data)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

GerbilManagerWebAPI/appsettings.Development.json

@@ -4,8 +4,5 @@
       "Default": "Information",
       "Microsoft.AspNetCore": "Warning"
     }
-  },
-  "ConnectionStrings": {
-    "sqlConnection": "server=.; database=GerbilManagerDebug; User Id=sa;Password=StrongerThenYYou1;Encrypt=False;TrustServerCertificate=True" 
   }
 }