fix(configuration-database): resolve ConfigurationDatabase-012 — store inbound-API keys as HMAC-SHA256 hashes

Inbound-API bearer credentials are no longer persisted in plaintext. ApiKey now
holds a KeyHash (peppered HMAC-SHA256); the key is shown once at creation and
only its hash is stored. Lookup and validation hash the presented candidate.
Cross-module: Commons (ApiKey, ApiKeyHasher), ConfigurationDatabase (mapping +
HashApiKeyValue migration), InboundAPI (ApiKeyValidator), ManagementService
(key creation), CentralUI (ApiKeys.razor). Existing keys must be re-issued.

src/ScadaLink.ConfigurationDatabase/Migrations/20260517073000_HashApiKeyValue.cs

@@ -0,0 +1,77 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ScadaLink.ConfigurationDatabase.Migrations
+{
+    /// <summary>
+    /// ConfigurationDatabase-012: replaces the plaintext <c>KeyValue</c> column with a
+    /// <c>KeyHash</c> column holding a deterministic HMAC-SHA256 hash of the key.
+    /// </summary>
+    /// <remarks>
+    /// A hash is one-way: existing plaintext keys cannot be converted to hashes
+    /// without the originals. This migration therefore deletes all existing API-key
+    /// rows. <strong>Every existing API key must be re-issued</strong> after this
+    /// migration is applied — create new keys via the CLI / Management API / Central
+    /// UI, distribute the one-time plaintext to callers, and approve them on methods.
+    /// </remarks>
+    public partial class HashApiKeyValue : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            // Existing keys hold only plaintext, which cannot be hashed back. They
+            // must be re-issued, so remove them before the column change to keep the
+            // new unique KeyHash index satisfiable.
+            migrationBuilder.Sql("DELETE FROM ApiKeys;");
+
+            migrationBuilder.DropIndex(
+                name: "IX_ApiKeys_KeyValue",
+                table: "ApiKeys");
+
+            migrationBuilder.DropColumn(
+                name: "KeyValue",
+                table: "ApiKeys");
+
+            migrationBuilder.AddColumn<string>(
+                name: "KeyHash",
+                table: "ApiKeys",
+                type: "nvarchar(256)",
+                maxLength: 256,
+                nullable: false,
+                defaultValue: "");
+
+            migrationBuilder.CreateIndex(
+                name: "IX_ApiKeys_KeyHash",
+                table: "ApiKeys",
+                column: "KeyHash",
+                unique: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropIndex(
+                name: "IX_ApiKeys_KeyHash",
+                table: "ApiKeys");
+
+            migrationBuilder.DropColumn(
+                name: "KeyHash",
+                table: "ApiKeys");
+
+            migrationBuilder.AddColumn<string>(
+                name: "KeyValue",
+                table: "ApiKeys",
+                type: "nvarchar(500)",
+                maxLength: 500,
+                nullable: false,
+                defaultValue: "");
+
+            migrationBuilder.CreateIndex(
+                name: "IX_ApiKeys_KeyValue",
+                table: "ApiKeys",
+                column: "KeyValue",
+                unique: true);
+        }
+    }
+}

src/ScadaLink.ConfigurationDatabase/Migrations/ScadaLinkDbContextModelSnapshot.cs

@@ -348,10 +348,10 @@ namespace ScadaLink.ConfigurationDatabase.Migrations
                     b.Property<bool>("IsEnabled")
                         .HasColumnType("bit");
 
-                    b.Property<string>("KeyValue")
+                    b.Property<string>("KeyHash")
                         .IsRequired()
-                        .HasMaxLength(500)
-                        .HasColumnType("nvarchar(500)");
+                        .HasMaxLength(256)
+                        .HasColumnType("nvarchar(256)");
 
                     b.Property<string>("Name")
                         .IsRequired()
@@ -360,7 +360,7 @@ namespace ScadaLink.ConfigurationDatabase.Migrations
 
                     b.HasKey("Id");
 
-                    b.HasIndex("KeyValue")
+                    b.HasIndex("KeyHash")
                         .IsUnique();
 
                     b.HasIndex("Name")