feat(rate-limiting): Implement core rate limiting functionality with configuration, decision-making, metrics, middleware, and service registration

- Add RateLimitConfig for configuration management with YAML binding support.
- Introduce RateLimitDecision to encapsulate the result of rate limit checks.
- Implement RateLimitMetrics for OpenTelemetry metrics tracking.
- Create RateLimitMiddleware for enforcing rate limits on incoming requests.
- Develop RateLimitService to orchestrate instance and environment rate limit checks.
- Add RateLimitServiceCollectionExtensions for dependency injection registration.

src/Signer/__Libraries/StellaOps.Signer.KeyManagement/KeyManagementDbContext.cs

@@ -0,0 +1,59 @@
+using Microsoft.EntityFrameworkCore;
+
+using StellaOps.Signer.KeyManagement.Entities;
+
+namespace StellaOps.Signer.KeyManagement;
+
+/// <summary>
+/// DbContext for key management entities.
+/// </summary>
+public class KeyManagementDbContext : DbContext
+{
+    public KeyManagementDbContext(DbContextOptions<KeyManagementDbContext> options)
+        : base(options)
+    {
+    }
+
+    /// <summary>
+    /// Key history entries.
+    /// </summary>
+    public DbSet<KeyHistoryEntity> KeyHistory => Set<KeyHistoryEntity>();
+
+    /// <summary>
+    /// Key audit log entries.
+    /// </summary>
+    public DbSet<KeyAuditLogEntity> KeyAuditLog => Set<KeyAuditLogEntity>();
+
+    /// <summary>
+    /// Trust anchors.
+    /// </summary>
+    public DbSet<TrustAnchorEntity> TrustAnchors => Set<TrustAnchorEntity>();
+
+    protected override void OnModelCreating(ModelBuilder modelBuilder)
+    {
+        base.OnModelCreating(modelBuilder);
+
+        modelBuilder.HasDefaultSchema("signer");
+
+        modelBuilder.Entity<KeyHistoryEntity>(entity =>
+        {
+            entity.HasKey(e => e.HistoryId);
+            entity.HasIndex(e => new { e.AnchorId, e.KeyId }).IsUnique();
+            entity.HasIndex(e => e.AnchorId);
+        });
+
+        modelBuilder.Entity<KeyAuditLogEntity>(entity =>
+        {
+            entity.HasKey(e => e.LogId);
+            entity.HasIndex(e => e.AnchorId);
+            entity.HasIndex(e => e.CreatedAt).IsDescending();
+        });
+
+        modelBuilder.Entity<TrustAnchorEntity>(entity =>
+        {
+            entity.HasKey(e => e.AnchorId);
+            entity.HasIndex(e => e.PurlPattern);
+            entity.HasIndex(e => e.IsActive);
+        });
+    }
+}