Fix migration strategy docs to match production

Coolify runs migrations post-deploy via rel/overlays/bin/migrate,
not on boot. Update both architecture and infrastructure docs.

Closes #141

docs/architecture.md

@@ -36,7 +36,7 @@ MusicLibrary.Application (one_for_one)
 │   ├── Telemetry.Storage        # Metrics storage (SQLite-backed, persistent)
 │   └── :telemetry_poller        # 30s periodic measurements
 ├── Oban                         # Background job engine
-├── Ecto.Migrator                # Auto-migration on boot
+├── Ecto.Migrator                # Migrations (skipped in release; run by Coolify post-deploy)
 ├── Task.Supervisor (MusicLibrary.TaskSupervisor)
 ├── Phoenix.PubSub (:music_library)
 └── MusicLibraryWeb.Endpoint

docs/production-infrastructure.md

@@ -164,7 +164,12 @@ A manual verification workflow (`.github/workflows/verify.yml`) can be triggered
 
 Standard Mix release. Entry point: `rel/overlays/bin/server` (sets `PHX_SERVER=true`).
 
-Migrations run automatically on boot via `Ecto.Migrator` in the supervision tree.
+Migrations are skipped during release boot (`skip_migrations?/0` returns `true` when
+`RELEASE_NAME` is set). Instead, Coolify is configured to run migrations after the
+Docker image is built and the container is started — this happens via the
+`rel/overlays/bin/migrate` script, executed as a post-deployment command in Coolify
+before the application begins serving traffic.
+
 A standalone `rel/overlays/bin/migrate` script is also available for manual use.
 
 ERL_FLAGS: `+JPperf true` (JIT performance monitoring).