fix(projection): serialise concurrent migrators with a pg advisory lock (refs #75)

verify-up failed: the event-subscriber and projection-api both migrate the shared
projection DB on start, and EF releases its migrations-history lock between individual
migrations — harmless with one migration, but the new AddNotificationResource made a
second, so a migrator re-applied it in the window between the other's two migrations
("column resource already exists"). Hold a session pg_advisory_lock across the whole
MigrateAsync so the sequence runs exactly once; the second migrator then finds nothing
pending.

Verified by starting both images simultaneously against a fresh Postgres: both reach
health, the resource column is created once, and __EFMigrationsHistory has both rows.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/projection-api/ProjectionApi.Api/Program.cs

@@ -10,8 +10,9 @@ builder.Services.AddProjectionReadModel(connectionString);
 
 var app = builder.Build();
 
-// Ensure the schema exists before serving reads. EF serialises concurrent migrators via the
-// migrations-history lock, so it is safe that the Event Subscriber migrates too.
+// Ensure the schema exists before serving reads. The Event Subscriber migrates this shared DB too;
+// MigrateProjectionAsync serialises concurrent migrators with a session advisory lock so the whole
+// migration sequence runs exactly once (see ServiceCollectionExtensions).
 await app.Services.MigrateProjectionAsync();
 
 app.MapGet("/health", () => "Healthy");