feat(tests): añade TestConnectionStrings y script de creación de DBs de test

Introduce SIGCM2_Test_App y SIGCM2_Test_Api como bases aisladas para
Application.Tests y Api.Tests respectivamente. TestConnectionStrings.cs
centraliza las connection strings; create-test-api-db.sql documenta
el setup idempotente de ambas bases con COLLATE Modern_Spanish_CI_AS.

database/init/create-test-api-db.sql

@@ -0,0 +1,30 @@
+-- create-test-api-db.sql
+-- Creates test databases for integration tests (idempotent).
+-- Run once per environment on TECNICA3 before executing integration tests.
+--
+-- SIGCM2_Test_App  -> used by SIGCM2.Application.Tests
+-- SIGCM2_Test_Api  -> used by SIGCM2.Api.Tests
+-- SIGCM2_Test      -> legacy (kept for old branches e.g. pre-merge CAT-001)
+--
+-- After creating the DBs, apply V010 to both new DBs:
+--   See database/README.md > "Test DBs" section for the PowerShell runbook.
+
+IF DB_ID(N'SIGCM2_Test_App') IS NULL
+BEGIN
+    CREATE DATABASE [SIGCM2_Test_App]
+        COLLATE Modern_Spanish_CI_AS;
+    PRINT 'Database SIGCM2_Test_App created.';
+END
+ELSE
+    PRINT 'Database SIGCM2_Test_App already exists -- skip.';
+GO
+
+IF DB_ID(N'SIGCM2_Test_Api') IS NULL
+BEGIN
+    CREATE DATABASE [SIGCM2_Test_Api]
+        COLLATE Modern_Spanish_CI_AS;
+    PRINT 'Database SIGCM2_Test_Api created.';
+END
+ELSE
+    PRINT 'Database SIGCM2_Test_Api already exists -- skip.';
+GO

tests/SIGCM2.TestSupport/TestConnectionStrings.cs

@@ -0,0 +1,20 @@
+using System.Runtime.CompilerServices;
+
+[assembly: InternalsVisibleTo("SIGCM2.Api.Tests")]
+
+namespace SIGCM2.TestSupport;
+
+/// <summary>
+/// Centralized connection string constants for integration test databases.
+/// Single source of truth — change server/credentials here only.
+/// </summary>
+public static class TestConnectionStrings
+{
+    /// <summary>Used by SIGCM2.Application.Tests via SqlTestFixture (parameterless ctor).</summary>
+    public const string AppTestDb =
+        "Server=TECNICA3;Database=SIGCM2_Test_App;User Id=desarrollo;Password=desarrollo2026;TrustServerCertificate=True;";
+
+    /// <summary>Used by SIGCM2.Api.Tests via TestWebAppFactory.</summary>
+    public const string ApiTestDb =
+        "Server=TECNICA3;Database=SIGCM2_Test_Api;User Id=desarrollo;Password=desarrollo2026;TrustServerCertificate=True;";
+}