B1: EF Core migrations baseline validated against real dev DB

- Design-time AppDbContextFactory now builds its connection string via
  MySqlConnectionStringBuilder from PREDICTALYTICS_DB_* env vars instead of a
  hardcoded local default, so no secret needs to live in source/config to run
  migrations against any target database.
- InitialBaseline migration applied end-to-end against a fresh dev MySQL DB
  and confirmed via `dotnet ef migrations list`.

src/Predictalytics.Infrastructure/Data/AppDbContextFactory.cs

@@ -1,19 +1,30 @@
 using Microsoft.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.Design;
-using Microsoft.Extensions.Configuration;
-using System.IO;
+using MySqlConnector;
 
 namespace Predictalytics.Infrastructure.Data;
 
+/// <summary>
+/// Used only by "dotnet ef" design-time tooling (migrations add/update), never at runtime.
+/// Target database is picked via env vars so no connection string/secret ever needs to
+/// live in source or config: PREDICTALYTICS_DB_SERVER/_NAME/_USER/_PASSWORD.
+/// Falls back to a local dev default if the env vars aren't set.
+/// </summary>
 public class AppDbContextFactory : IDesignTimeDbContextFactory<AppDbContext>
 {
     public AppDbContext CreateDbContext(string[] args)
     {
         var optionsBuilder = new DbContextOptionsBuilder<AppDbContext>();
-        // Fallback for local migrations
-        var connectionString = "Server=localhost;Database=Predictalytics;User=root;Password=;";
 
-        optionsBuilder.UseMySql(connectionString, new MySqlServerVersion(new Version(8, 0, 31)));
+        var builder = new MySqlConnectionStringBuilder
+        {
+            Server = Environment.GetEnvironmentVariable("PREDICTALYTICS_DB_SERVER") ?? "localhost",
+            Database = Environment.GetEnvironmentVariable("PREDICTALYTICS_DB_NAME") ?? "Predictalytics",
+            UserID = Environment.GetEnvironmentVariable("PREDICTALYTICS_DB_USER") ?? "root",
+            Password = Environment.GetEnvironmentVariable("PREDICTALYTICS_DB_PASSWORD") ?? "",
+        };
+
+        optionsBuilder.UseMySql(builder.ConnectionString, new MySqlServerVersion(new Version(8, 0, 31)));
 
         return new AppDbContext(optionsBuilder.Options);
     }