feat(work-orders): add board flagColor with fixed palette

Support PATCH flagColor with FE-aligned #RRGGBB palette and expose the
value on board/detail rows.

Data.SeaHavenIndustries/Migrations/20260715180000_WoFlagColor.cs

@@ -0,0 +1,33 @@
+using Data.SeaHavenIndustries;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Data.SeaHavenIndustries.Migrations
+{
+    /// <inheritdoc />
+    [DbContext(typeof(ApplicationDbContext))]
+    [Migration("20260715180000_WoFlagColor")]
+    public partial class WoFlagColor : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "FlagColor",
+                table: "workOrders",
+                type: "nvarchar(7)",
+                maxLength: 7,
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "FlagColor",
+                table: "workOrders");
+        }
+    }
+}

SeaHaven.Services/Helpers/WorkOrderFlagColors.cs

@@ -0,0 +1,50 @@
+namespace SeaHaven.Services.Helpers
+{
+    /// <summary>
+    /// Fixed board flag palette (must match FE FLAG_COLORS). Values stored as canonical #RRGGBB.
+    /// </summary>
+    public static class WorkOrderFlagColors
+    {
+        public static readonly HashSet<string> Allowed = new(StringComparer.OrdinalIgnoreCase)
+        {
+            "#EF4444",
+            "#F97316",
+            "#F59E0B",
+            "#EAB308",
+            "#22C55E",
+            "#14B8A6",
+            "#3B82F6",
+            "#8B5CF6"
+        };
+
+        /// <summary>
+        /// Normalizes to # + 6 uppercase hex digits, or null if empty.
+        /// Returns false if the value is non-empty but not a valid hex color.
+        /// </summary>
+        public static bool TryNormalize(string? value, out string? canonical)
+        {
+            canonical = null;
+            if (string.IsNullOrWhiteSpace(value))
+                return true;
+
+            var trimmed = value.Trim();
+            if (trimmed.Length == 6 && trimmed[0] != '#')
+                trimmed = "#" + trimmed;
+
+            if (trimmed.Length != 7 || trimmed[0] != '#')
+                return false;
+
+            for (var i = 1; i < 7; i++)
+            {
+                if (!Char.IsAsciiHexDigit(trimmed[i]))
+                    return false;
+            }
+
+            canonical = "#" + trimmed[1..].ToUpperInvariant();
+            return true;
+        }
+
+        public static bool IsAllowed(string? canonical)
+            => canonical != null && Allowed.Contains(canonical);
+    }
+}