test(db): run the startup migration chain against PostgreSQL in CI

Every PostgreSQL test carries Category=RequiresDocker and the pipeline filters
that category out, so the provider production runs on was never exercised and
two upgrade-path bugs reached it in a row. Nothing covered the whole startup
sequence either - code and schema migrations interleaved the way the migration
service orders them.

- run both database stages through JellyfinMigrationService against a seeded
  library and against a fresh install
- take the server from JELLYFIN_TEST_POSTGRES when it is set, else start a
  container
- add a pipeline step that runs the PostgreSQL tests on every push and pull
  request, with the server inside the step: the kubernetes backend has no
  docker daemon, and a service container deadlocks on the workspace volume

README.md

@@ -461,6 +461,18 @@ dotnet test Jellyfin.sln \
   --filter "Category!=RequiresDocker&FullyQualifiedName!~Integration"
 ```
 
+### Run the PostgreSQL migration tests
+
+They run the startup migration chain against a real server. Set `JELLYFIN_TEST_POSTGRES` to a
+connection string for an already running server and they use it; without it they start a container
+through testcontainers.
+
+```bash
+dotnet test tests/Jellyfin.Server.Tests \
+  --configuration Release \
+  --filter "Category=RequiresDocker"
+```
+
 ### Run HA-specific tests
 
 The transcode session store and HA recovery tests live in: