feat(projection): persist the read projection and expose webhook + read APIs (refs #7)

Add the projection persistence and the two services around it:

- Projection.ReadModel: a shared EF Core (Npgsql) read model owning the projection
  schema — register_projection + the subscriber's processed_notifications log — plus
  EfProjectionStore / EfNotificationLog (atomic record-or-skip on the PK for idempotency)
  and the initial migration. One rebuildable store, written by the subscriber and read
  by projection-api (ADR-0008).
- EventSubscriber.Api: POST /notifications NRC callback (enforces the abonnement bearer,
  401 without it per ADR-0007), POST /admin/rebuild, /health. Migrates on start.
- ProjectionApi.Api: GET /register, GET /register/{id}, /health — the read side.

dotnet-ef pinned as a local tool for migrations; NuGetAuditMode=direct so EF's
design-time-only tooling transitive doesn't flag the shipped build.

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

services/projection-api/Projection.ReadModel/DesignTimeProjectionDbContextFactory.cs

@@ -0,0 +1,18 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Design;
+
+namespace Projection.ReadModel;
+
+/// <summary>Lets <c>dotnet ef migrations</c> build the context at design time without a running
+/// database or the host's DI. The connection string is a placeholder — migrations only need the
+/// provider to emit Postgres-shaped SQL.</summary>
+public sealed class DesignTimeProjectionDbContextFactory : IDesignTimeDbContextFactory<ProjectionDbContext>
+{
+    public ProjectionDbContext CreateDbContext(string[] args)
+    {
+        var options = new DbContextOptionsBuilder<ProjectionDbContext>()
+            .UseNpgsql("Host=localhost;Database=projection;Username=projection;Password=projection")
+            .Options;
+        return new ProjectionDbContext(options);
+    }
+}