chore(backend): add explicit test data seed command

docs/TASKS/development/001-script-testdata-seed.md

@@ -0,0 +1,20 @@
+# Task: Add explicit test data seed script
+
+## Goal
+
+Move demo/test data seeding out of API startup and into an explicit script command that can be run intentionally against any configured environment.
+
+## Relevant Files
+
+- `backend/src/Socialize.Api/Program.cs`
+- `backend/src/Socialize.Api/Infrastructure/TestData/TestDataSeedExtensions.cs`
+- `backend/src/Socialize.Api/appsettings.Development.json`
+- `backend/src/Socialize.Api/appsettings.Production.json`
+- `scripts/seed-testdata.sh`
+
+## Validation
+
+```bash
+dotnet build backend/Socialize.slnx
+./scripts/seed-testdata.sh
+```